php: fix typo

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/php/php.mk

@@ -126,7 +126,7 @@ ifeq ($(BR2_PACKAGE_PHP_EXT_INTL),y)
 	# The intl module is implemented in C++, but PHP fails to use
 	# g++ as the compiler for the final link. As a workaround,
 	# tell it to link libstdc++.
-	PHP_EXTRA_LIBS + = -lstdc++
+	PHP_EXTRA_LIBS += -lstdc++
 endif
 
 ifeq ($(BR2_PACKAGE_PHP_EXT_GMP),y)