package/exiv2: fix build without SSP

package/exiv2: fix build without SSP

Build without SSP fails since bump to version 0.27.4 in commit
bcace429426ee91aac56f3dcc33b69e22141d384

This is due to the fact that
https://github.com/Exiv2/exiv2/commit/bbe0b70840cf28b7dd8c0b7e9bb1b741aeda2efd
removed the wrong GCC_ prefix from HAS_FSTACK_PROTECTOR_STRONG variable

Fixes:
 - http://autobuild.buildroot.org/results/ae4635899124c602c70d2b342a76f95c34aa4a3d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>