package/libselinux: fix glibc build with old toolchains

package/libselinux: fix glibc build with old toolchains

For an unknown reason, since bump to version 3.0.0, we got build failure
due to -D_FILE_OFFSET_BITS==64:

/home/naourr/work/instance-2/output-1/host/arm-buildroot-linux-gnueabi/sysroot/usr/include/fts.h:41:3: error: #error "<fts.h> cannot be used with -D_FILE_OFFSET_BITS==64"
 # error "<fts.h> cannot be used with -D_FILE_OFFSET_BITS==64"
   ^

Update our workaround to also filter CPPFLAGS fix the issue and seems
right as we're doing it for all the other affected packages
(restorecond, elfutils ...)

Fixes:
 - http://autobuild.buildroot.org/results/200fd0accf6a1926251243b05e600fbf591bb3a2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>