Fix std::unordered_set declaration

[79/471] Building CXX object src/libslic3r/CMakeFiles/libslic3r.dir/FaceDetector.cpp.o
FAILED: src/libslic3r/CMakeFiles/libslic3r.dir/FaceDetector.cpp.o
/run/ccache/bin/c++ -DBOOST_ATOMIC_NO_LIB -DBOOST_CHRONO_NO_LIB -DBOOST_DATE_TIME_NO_LIB -DBOOST_FILESYSTEM_NO_LIB -DBOOST_IOSTREAMS_NO_LIB -DBOOST_LOCALE_NO_LIB -DBOOST_LOG_NO_LIB -DBOOST_REGEX_NO_LIB -DBOOST_SYSTEM_NO_LIB -DBOOST_THREAD_NO_LIB -DHAVE_FREETYPE -DHAVE_OPENGL_EXT -DHAVE_XLIB -DLIBNEST2D_GEOMETRIES_libslic3r -DLIBNEST2D_OPTIMIZER_nlopt -DLIBNEST2D_STATIC -DLIBNEST2D_THREADING_tbb -DOCC_CONVERT_SIGNALS -DOPENVDB_OPENEXR_STATICLIB -DOPENVDB_STATICLIB -DSLIC3R_GUI -DTBB_USE_CAPTURED_EXCEPTION=0 -DUNICODE -DUSE_TBB -DWXINTL_NO_GETTEXT_MACRO -D_UNICODE -DwxNO_UNSAFE_WXSTRING_CONV -DwxUSE_UNICODE -I/usr/include/dbus-1.0 -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include -I/run/build/BambuStudio/src -I/run/build/BambuStudio/build/src/platform -I/run/build/BambuStudio/src/libslic3r -I/run/build/BambuStudio/build/src/libslic3r -I/run/build/BambuStudio/deps/build/destdir/usr/local/include/opencascade -I/run/build/BambuStudio/src/libnest2d/include -I/run/build/BambuStudio/src/miniz -I/run/build/BambuStudio/src/glu-libtess/include -I/run/build/BambuStudio/src/clipper2/Clipper2Lib/include -isystem /run/build/BambuStudio/src/eigen -isystem /run/build/BambuStudio/src/libigl -isystem /run/build/BambuStudio/deps/build/destdir/usr/local/include -isystem /run/build/BambuStudio/deps/build/destdir/usr/local/include/boost-1_78 -isystem /run/build/BambuStudio/deps/build/destdir/usr/local/include/OpenEXR -O2 -pipe -g -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fno-omit-frame-pointer  -std=gnu++11 -fext-numeric-literals -Wall -Wno-reorder -O3 -DNDEBUG -std=gnu++17 -fPIC -fsigned-char -Werror=return-type -Wno-ignored-attributes -Wno-unknown-pragmas -DOPENVDB_ABI_VERSION_NUMBER=8 -MD -MT src/libslic3r/CMakeFiles/libslic3r.dir/FaceDetector.cpp.o -MF src/libslic3r/CMakeFiles/libslic3r.dir/FaceDetector.cpp.o.d -o src/libslic3r/CMakeFiles/libslic3r.dir/FaceDetector.cpp.o -c /run/build/BambuStudio/src/libslic3r/FaceDetector.cpp
/run/build/BambuStudio/src/libslic3r/FaceDetector.cpp:33:10: error: ‘unordered_set’ is not a member of ‘std’
   33 |     std::unordered_set<size_t> hit_face_indices;
      |          ^~~~~~~~~~~~~
/run/build/BambuStudio/src/libslic3r/FaceDetector.cpp:5:1: note: ‘std::unordered_set’ is defined in header ‘<unordered_set>’; did you forget to ‘#include <unordered_set>’?
    4 | #include "Model.hpp"
  +++ |+#include <unordered_set>
    5 |

src/libslic3r/FaceDetector.cpp

@@ -2,6 +2,7 @@
 #include "TriangleMesh.hpp"
 #include "SLA/IndexedMesh.hpp"
 #include "Model.hpp"
+#include <unordered_set>
 
 namespace Slic3r {
 static const double BBOX_OFFSET = 2.0;