Remove EXTERN_C and related defines

common-defs.h has a few defines that I suspect were used during the
transition to C++.  These aren't needed any more, so remove them.

Tested by rebuilding.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/defs.h

@@ -303,7 +303,7 @@ extern int annotation_level;	/* in stack.c */
    "const char *" in unistd.h, so we can't declare the argument
    as "char *".  */
 
-EXTERN_C char *re_comp (const char *);
+extern "C" char *re_comp (const char *);
 
 /* From symfile.c */