uml: PATH_MAX needs limits.h

Include limits.h to get a definition of PATH_MAX.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>