kbuild: fix $(src) assignmnet with external modules

When we introduced support for KBUILD_EXTRA_SYMBOLS
we started to include the externam module's kbuild
file when doing the final modpost step.

As external modules often do:
ccflags-y := -I$(src)

We had problems because $(src) was unassinged and
gcc then used the next parameter for -I resulting in
strange build failures.

Fix is to assign $(src) and $(obj) when building
external modules.

This fixes: http://bugzilla.kernel.org/show_bug.cgi?id=10798

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Tvrtko <tvrtko.ursulin@sophos.com>
Cc: Andrea Arcangeli <andrea@qumranet.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>