um: tell git to ignore generated files

Tell git to ignore the generated files under um, except:

 include/shared/kern_constants.h
 include/shared/user_constants.h

which will be moved to include/generated.

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>