kbuild: Fix <linux/version.h> for empty SUBLEVEL or PATCHLEVEL

expr treats all numbers as decimals, so prepending a zero is safe. Note
that the KERNEL_VERSION() macro still takes three arguments, 3.0 has to be
written as KERNEL_VERSION(3,0,0).
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -1005,7 +1005,7 @@ endef
 
 define filechk_version.h
 	(echo \#define LINUX_VERSION_CODE $(shell                             \
-	expr $(VERSION) \* 65536 + $(PATCHLEVEL) \* 256 + $(SUBLEVEL));     \
+	expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 0$(SUBLEVEL));    \
 	echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))';)
 endef