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>
Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>

tools/pkg-config/patches/001-glib-gdate-suppress-string-format-literal-warning.patch

@@ -0,0 +1,18 @@
+--- a/glib/glib/gdate.c
++++ b/glib/glib/gdate.c
+@@ -2439,6 +2439,9 @@ win32_strftime_helper (const GDate     *d,
+  *
+  * Returns: number of characters written to the buffer, or 0 the buffer was too small
+  */
++#pragma GCC diagnostic push
++#pragma GCC diagnostic ignored "-Wformat-nonliteral"
++
+ gsize     
+ g_date_strftime (gchar       *s, 
+                  gsize        slen, 
+@@ -2549,3 +2552,5 @@ g_date_strftime (gchar       *s,
+   return retval;
+ #endif
+ }
++
++#pragma GCC diagnostic pop