more missing includes (#5930)

Here are more missing header files recently identified by building
without precompiled header support.

src/libslic3r/GCode/SmallAreaInfillFlowCompensator.cpp

@@ -9,6 +9,7 @@
 #include <math.h>
 #include <cstring>
 #include <cfloat>
+#include <regex>
 
 #include "../libslic3r.h"
 #include "../PrintConfig.hpp"