package/libexif: fix build with gcc 4.8

package/libexif: fix build with gcc 4.8

Fix the following build failure with gcc 4.8 raised since bump to
version 0.6.23 in commit e2f805097611b4828d2cba6168472aac6dedeafe:

exif-gps-ifd.c: In function 'exif_get_gps_tag_info':
exif-gps-ifd.c:62:3: error: 'for' loop initial declarations are only allowed in C99 mode
   for (int i = 0; i < sizeof(exif_gps_ifd_tags) / sizeof(ExifGPSIfdTagInfo); ++i) {
   ^
exif-gps-ifd.c:62:3: note: use option -std=c99 or -std=gnu99 to compile your code

Fixes:
 - http://autobuild.buildroot.org/results/7dd222e06d1e6611449fb8fe7516817c9ad43d65

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>