php7-pecl-http: give hint about zlib staging dir

Buildbots currently fail to find zlib souces, so try to give
this hint with configure parameters.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

lang/php7-pecl-http/Makefile

@@ -49,6 +49,7 @@ CONFIGURE_VARS+= \
 CONFIGURE_ARGS+= \
 	--with-http \
 	--without-http-shared-deps \
+	--with-http-zlib-dir="$(STAGING_DIR)/usr" \
 	--with-http-libcurl-dir="$(STAGING_DIR)/usr" \
 	--with-http-libevent-dir="$(STAGING_DIR)/usr" \
 	--with-http-libidn-dir="$(STAGING_DIR)/usr" \