add missing header stdlib.h for exit

Geo/GEdgeCompound.cpp

@@ -10,6 +10,7 @@
 #include "GmshConfig.h"
 #include "GEdgeCompound.h"
 #include "Numeric.h"
+#include "stdlib.h"
 
 GEdgeCompound::GEdgeCompound(GModel *m, int tag, std::vector<GEdge*> &compound)
   : GEdge(m, tag, 0 , 0), _compound(compound)