Fixes this in C++ mode:

  src/gdb/python/python-internal.h: At global scope:
  src/gdb/python/python-internal.h:313:13: error: use of enum ‘ext_lang_rc’ without previous declaration
   extern enum ext_lang_rc gdbpy_apply_val_pretty_printer
	       ^
  src/gdb/python/python-internal.h:320:41: error: invalid type in declaration before ‘;’ token
      const struct language_defn *language);
					   ^

gdb/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* python/python-internal.h: Include "extension-priv.h".