Fixes build with newer gcc

gdate.c: In function ‘g_date_strftime’:
gdate.c:2497:7: error: format not a string literal, format string not checked [-Werror=format-nonliteral]
       tmplen = strftime (tmpbuf, tmpbufsize, locale_format, &tm);
       ^~~~~~
cc1: some warnings being treated as errors
make[6]: *** [Makefile:1387: libglib_2_0_la-gdate.lo] Error 1