UAPI: Strip _UAPI prefix on header install no matter the whitespace

Commit 56c176c9 ("UAPI: strip the _UAPI prefix from header guards
during header installation") strips the _UAPI prefix from header guards,
but only if there's a single space between the cpp directive and the
label.

Make it more flexible and able to handle tabs and multiple white space
characters.

Signed-off-by: David Howells <dhowell@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>