kbuild: remove duplicate, conflicting entry for oom.h

oom.h is already tagged for unifdef'ing, so its entry as a simple exportable
header should be deleted.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>