prot_hsymlinks.c: Define _GNU_SOURCE which is required for O_DIRECTORY

O_DIRECTORY is only available if _GNU_SOURCE is defined
in uClibc toolchains.

This is similar to 0d0c695c8c6657710d1a8ecb4f1264dcd405276b
"open11.c: Define _GNU_SOURCE which is required for O_DIRECTORY"

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>