jool: replace SUBDIRS with M in package recipe

The SUBDIRS variable has been removed in kernel 5.4, and was deprecated
since the beginnig of kernel git history in favour of M or KBUILD_EXTMOD.

Signed-off-by: David Bauer <mail@david-bauer.net>

net/jool/Makefile

@@ -30,21 +30,21 @@ define Build/Compile
 		KERNEL_DIR="$(LINUX_DIR)" \
 		ARCH="$(LINUX_KARCH)" \
 		CROSS_COMPILE="$(TARGET_CROSS)" \
-		SUBDIRS="$(PKG_BUILD_DIR)/src/mod/common" \
+		M="$(PKG_BUILD_DIR)/src/mod/common" \
 		V="$(V)" \
 		modules
 	$(MAKE) -C "$(LINUX_DIR)" \
 		KERNEL_DIR="$(LINUX_DIR)" \
 		ARCH="$(LINUX_KARCH)" \
 		CROSS_COMPILE="$(TARGET_CROSS)" \
-		SUBDIRS="$(PKG_BUILD_DIR)/src/mod/nat64" \
+		M="$(PKG_BUILD_DIR)/src/mod/nat64" \
 		V="$(V)" \
 		modules
 	$(MAKE) -C "$(LINUX_DIR)" \
 		KERNEL_DIR="$(LINUX_DIR)" \
 		ARCH="$(LINUX_KARCH)" \
 		CROSS_COMPILE="$(TARGET_CROSS)" \
-		SUBDIRS="$(PKG_BUILD_DIR)/src/mod/siit" \
+		M="$(PKG_BUILD_DIR)/src/mod/siit" \
 		V="$(V)" \
 		modules
 	$(call Build/Compile/Default)