php: use $() to reference make variables instead of ${}

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit dba254cdf006e559ed61e1a136677843a3b4a94e)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/php/php.mk

@@ -137,7 +137,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_PHP_EXT_LIBXML2),y)
 PHP_CONF_ENV += php_cv_libxml_build_works=yes
-PHP_CONF_OPTS += --enable-libxml --with-libxml-dir=${STAGING_DIR}/usr
+PHP_CONF_OPTS += --enable-libxml --with-libxml-dir=$(STAGING_DIR)/usr
 PHP_DEPENDENCIES += libxml2
 endif