package/procps-ng: install libprocps.pc in staging/usr/lib/pkgconfig/

package/procps-ng: install libprocps.pc in staging/usr/lib/pkgconfig/

Since 834893a965a993e8a73e30b448426f626419f3c8, pkg-config --libs libprocps
doesn't work anymore because libprocps.pc is installed in
STAGING_DIR/lib/pkgconfig/ instead of STAGING_DIR/usr/lib/pkgconfig/.

Add --libdir=/usr/lib in PROCPS_NG_CONF_OPTS to fixes the issue.
Note that libprocps.so is now installed in /usr/lib/ instead of /lib/.

Fixes:
http://autobuild.buildroot.net/results/2e5/2e5100ccf1ed336c26b83930e7149941c97fc4d2/

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