modphp: fix rpath QA issue

Fixes the following QA issue:

ERROR: QA Issue: package modphp contains bad RPATH ${STAGING_DIR_TARGET}/usr/lib in file ${WORKDIR}/packages-split/modphp/usr/lib/apache2/modules/libphp5.so

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>