openvswitch: use EXTRA_CFLAGS instead of KCFLAGS

The patch we've switched requires EXTRA_CFLAGS to be set instead of KCFLAGS.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

net/openvswitch/Makefile

@@ -136,7 +136,7 @@ define Build/Compile
 		ARCH="$(LINUX_KARCH)" \
 		SUBDIRS="$(PKG_BUILD_DIR)/datapath/linux" \
 		PATH="$(TARGET_PATH)" \
-		KCFLAGS="$(KCFLAGS)" \
+		EXTRA_CFLAGS="$(KCFLAGS)" \
 		KCC="$(KERNEL_CC)"
 endef