Move mrb_name_error() declaration to mruby.h As it is requred by struct mrbgem.

include/mruby.h

@@ -282,6 +282,7 @@ void mrb_exc_raise(mrb_state *mrb, mrb_value exc);
 
 void mrb_raise(mrb_state *mrb, struct RClass *c, const char *msg);
 void mrb_raisef(mrb_state *mrb, struct RClass *c, const char *fmt, ...);
+void mrb_name_error(mrb_state *mrb, mrb_sym id, const char *fmt, ...);
 void mrb_warn(const char *fmt, ...);
 void mrb_bug(const char *fmt, ...);
 

src/error.h

@@ -13,7 +13,6 @@ mrb_value mrb_exc_new3(mrb_state *mrb, struct RClass* c, mrb_value str);
 mrb_value make_exception(mrb_state *mrb, int argc, mrb_value *argv, int isstr);
 mrb_value mrb_make_exception(mrb_state *mrb, int argc, mrb_value *argv);
 mrb_value mrb_format(mrb_state *mrb, const char *format, ...);
-void mrb_name_error(mrb_state *mrb, mrb_sym id, const char *fmt, ...);
 void mrb_exc_print(mrb_state *mrb, struct RObject *exc);
 
 #endif  /* MRUBY_ERROR_H */