tools: pkg-config: fix build with GCC 6

Fixes the following error:

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);
       ^~~~~~

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>