Gmt module: Prevent conflict of definitions of yyguts_t from different lex files.

modules/gmt/gmt_filter_headfoot.l

@@ -16,6 +16,8 @@
 #define register
 // Get rid of warning on unused function
 #define YY_NO_INPUT
+// struct yyguts_t defined in each flex output file, this emits warning at linking stage
+#define yyguts_t gmt_filter_headfootguts_t
 #define yyterminate {return 0;}
 %}