info: fix compiler warning with -Wpedantic and gcc 5

info: fix compiler warning with -Wpedantic and gcc 5

Gstreamer compiled with gcc 5.2 and -Wpedantic produces the
following warning:

'ISO C does not support '__FUNCTION__' predefined identifier [-Wpedantic]
  const char *s = __FUNCTION__;'

Since gcc 5 enables C99 by default, use __func__ if it's available
instead of the non-standard __FUNCTION__ (as suggested in [2]).

[1]: https://gcc.gnu.org/gcc-5/changes.html
[2]: https://gcc.gnu.org/gcc-5/porting_to.html

https://bugzilla.gnome.org/show_bug.cgi?id=758541