kbuild: re-export VERSION, PATCHLEVEL, SUBLEVEL

Eric Sandeen <sandeen@sgi.com> pointed out that it is usefull to have
access to VERSION, PATCHLEVEL, SUBLEVEL in external modules, and gooling
a litte confirmed this.
So re-export them.
Usage within the kernel is still discouraged but possible.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -338,7 +338,7 @@ AFLAGS		:= -D__ASSEMBLY__
 # Read KERNELRELEASE from .kernelrelease (if it exists)
 KERNELRELEASE = $(shell cat .kernelrelease 2> /dev/null)
 
-export	KERNELRELEASE \
+export	VERSION PATCHLEVEL SUBLEVEL KERNELRELEASE \
 	ARCH CONFIG_SHELL HOSTCC HOSTCFLAGS CROSS_COMPILE AS LD CC \
 	CPP AR NM STRIP OBJCOPY OBJDUMP MAKE AWK GENKSYMS PERL UTS_MACHINE \
 	HOSTCXX HOSTCXXFLAGS LDFLAGS_MODULE CHECK CHECKFLAGS