package/unbound: fix detection of libexpat

package/unbound: fix detection of libexpat

unbound open-codes the detection of libexpat, and as all packages doing
so, look in host paths (/usr/local /opt/local /usr/lib /usr/pkg /usr/sfw
/usr). Obviously this is wrong for cross-compilation.

Do for libexpat as we do for openssl, and point unbound to staging dir,
when we know expat is.

Fixes:
    http://autobuild.buildroot.org/results/a89ea6aa1cd8c253b1260ea227fea3dc9b095fe8/

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Stefan Ott <stefan@ott.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>