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>