[PATCH] uml: add dependency to arch/um/Makefile for parallel builds

the header file must be build before mk_user_constants.  Adding it as a
direct dep doesnt work for some reason.

arch/um/os-Linux/util/mk_user_constants.c:2:26: error: user-offsets.h: No such file or directory
arch/um/os-Linux/util/mk_user_constants.c: In function 'main':
arch/um/os-Linux/util/mk_user_constants.c:17: error: '__UM_FRAME_SIZE' undeclared (first use in this function)
arch/um/os-Linux/util/mk_user_constants.c:17: error: (Each undeclared identifier is reported only once
arch/um/os-Linux/util/mk_user_constants.c:17: error: for each function it appears in.)
make[1]: *** [arch/um/os-Linux/util/mk_user_constants] Error 1

Signed-off-by: Olaf Hering <olh@suse.de>
Cc: Paolo Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>