siit: 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/siit/Makefile

@@ -36,7 +36,7 @@ define Build/Prepare
 endef
 
 define Build/Compile
-	$(MAKE) $(KERNEL_MAKEOPTS) SUBDIRS="$(PKG_BUILD_DIR)" modules
+	$(MAKE) $(KERNEL_MAKEOPTS) M="$(PKG_BUILD_DIR)" modules
 endef
 
 $(eval $(call KernelPackage,siit))