uml: correctly strip kernel defines from userspace CFLAGS

KERNEL_DEFINES needs whitespace trimmed, otherwise the whitespace crunching
done by make fools the patsubst which is used to remove KERNEL_DEFINES from
USER_CFLAGS.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>