Fix compile error under VS2017 (/Zc:strictStrings)

gmsrc/src/gm/gmCodeTree.cpp

@@ -545,7 +545,7 @@ void gmProcessDoubleQuoteString(char * a_string)
 
 
 
-int gmerror(char * a_message)
+int gmerror(const char * a_message)
 {
   gmCodeTree & ct = gmCodeTree::Get();
   if(ct.GetLog())

gmsrc/src/gm/gmCodeTree.h

@@ -253,7 +253,7 @@ struct gmCodeTreeNode
 
 void gmProcessSingleQuoteString(char * a_string);
 void gmProcessDoubleQuoteString(char * a_string);
-int gmerror(char * a_message);
+int gmerror(const char * a_message);
 int gmparse(void);
 
 #endif // _GMCODETREE_H_