kbuild: fix a cscope bug (make cscope segfaults)

Workaround a cscope bug where a trailing ':' in VPATH makes it segfault
and let it build the cross-reference succesfully.

VPATH=/home/mattia/devel/kernel/git/linux-2.6: cscope -b
[1]    17555 segmentation fault VPATH=/home/mattia/devel/kernel/git/linux-2.6: cscope -b
Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -137,7 +137,7 @@ objtree		:= $(CURDIR)
 src		:= $(srctree)
 obj		:= $(objtree)
 
-VPATH		:= $(srctree):$(KBUILD_EXTMOD)
+VPATH		:= $(srctree)$(if $(KBUILD_EXTMOD),:$(KBUILD_EXTMOD))
 
 export srctree objtree VPATH TOPDIR