do not pollute global namespace by including mscg header in fix header

src/MSCG/fix_mscg.cpp

@@ -19,6 +19,7 @@
 #include <string.h>
 #include <stdlib.h>
 #include "fix_mscg.h"
+#include "mscg.h"
 #include "atom.h"
 #include "domain.h"
 #include "error.h"

src/MSCG/fix_mscg.h

@@ -21,7 +21,6 @@ FixStyle(mscg,FixMSCG)
 #define LMP_FIX_MSCG_H
 
 #include "fix.h"
-#include "mscg.h"
 
 namespace LAMMPS_NS {