package/php: fix building pcre extension

package/php: fix building pcre extension

The configure option "--with-pcre-regex=$(STAGING_DIR)/usr" is broken.
PHP will  prepend $(STAGING_DIR)/usr to the paths, which will cause a
failure because it won't be able to find pcre, and will then fallback to
searching for pcre2, which won't be installed.

Removing "=$(STAGING_DIR)/usr" from --with-pcre-regex fixes the issue.

Fixes:
  http://autobuild.buildroot.net/results/586/586f56e8fcf2d2bbbd3bdf69b1c3befff7ce8bbf
Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Tested-by: Mark Corbin <mark.corbin@embecosm.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>