kbuild: fix KBUILD_EXTRA_SYMBOLS

Taken from http://bugzilla.kernel.org/show_bug.cgi?id=11567

If you even define KBUILD_EXTRA_SYMBOLS in Makefile it will not be expanded
into command line argument for modpost.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>