Staging: workaround build system bug

This is needed as CONFIG_STAGING is set to y, yet there is no code in
drivers/staging/ to build, so the build-in.o doesn't get created
properly.  Create a "dummy" module in drivers/staging called staging.c
to work around this bug.

Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>