package/php: fix building curl extension

package/php: fix building curl extension

The configure option "--with-curl=$(STAGING_DIR)/usr" is broken.
PHP will detect libcurl.pc, which will pass the configure checks, but will then
prepend $(STAGING_DIR)/usr to the paths in libcurl.pc.

Thus php will then search $(STAGING_DIR)/usr/$(STAGING_DIR)/usr/lib/ for
curl libraries during linking, which causes linking errors.

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

Fixes:

  http://autobuild.buildroot.net/results/44b9ea1edca85b222a117a8e241a26b8dce33929/

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>