package/gdb: fix build with glibc 2.20

Backport a patch from gdb 7.8.

Fixes:
http://autobuild.buildroot.net/results/3d5/3d5cb8bea0964f0d46733e47d04a053021e63643/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>