matlab 2018 uses strange macro, include first

Former-commit-id: 01ed8dedbb3d3f21a437ea756bb6ef8a6c80b249

include/igl/matlab/mexErrMsgTxt.cpp

@@ -7,9 +7,6 @@
 // obtain one at http://mozilla.org/MPL/2.0/.
 #include "mexErrMsgTxt.h"
 
-// Overload mexErrMsgTxt to check an assertion then print text only if
-// assertion fails
-#include "mex.h"
 IGL_INLINE void igl::matlab::mexErrMsgTxt(bool assertion, const char * text)
 {
   if(!assertion)

include/igl/matlab/mexErrMsgTxt.h

@@ -8,6 +8,9 @@
 #ifndef IGL_MATLAB_MEXERRMSGTXT_H
 #define IGL_MATLAB_MEXERRMSGTXT_H
 #include "../igl_inline.h"
+// Overload mexErrMsgTxt to check an assertion then print text only if
+// assertion fails
+#include "mex.h"
 namespace igl
 {
   namespace matlab