gnutls: Point to staging area for finding seccomp libs and includes

This ensures that if libseccomp is installed on build host then it does
not resort to use it.

Fixes
checking for libseccomp... (cached) yes
checking how to link with libseccomp... /usr/lib/libseccomp.so

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>