libstrophe: fix build with full language support enabled

After d18692c, we need to include nls.mk to setup correct
environment variables so that linking succeeds.

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

libs/libstrophe/Makefile

@@ -28,6 +28,7 @@ PKG_CONFIG_DEPENDS:= \
 	CONFIG_libstrophe-expat
 
 include $(INCLUDE_DIR)/package.mk
+include $(INCLUDE_DIR)/nls.mk
 
 define Package/libstrophe
 	SECTION:=libs