FIX: fix a typo in GCodeProcessor

Change-Id: I56b9eaedc3cb062d17facf7352eb469524db5b60
(cherry picked from commit 1bb3b64cfc1aabad9ece3e5d5c0f55a9cb3367c5)

src/libslic3r/GCode/GCodeProcessor.cpp

@@ -1683,7 +1683,7 @@ void GCodeProcessor::process_gcode_line(const GCodeReader::GCodeLine& line, bool
                 case '2':
                 case '3': { process_G2_G3(line); break; }  // Move
                 //BBS
-                case 4:  { process_G4(line); break; }  // Delay
+                case '4':  { process_G4(line); break; }  // Delay
                 default: break;
                 }
                 break;