prebuilts: Disable _FORTIFY_SOURCE for native libselinux

This patch disables the macro _FORTIFY_SOURCE for libselinux native
compilation with gcc. This fixes the following error:
 "inlining failed in call to always_inline ‘syslog’: function body can be overwritten at link time".

Signed-off-by: Thierry Escande <thierry.escande@huawei.com>