[PATCH] Permit multiple inclusion of linux/pagevec.h

Make it possible to include linux/pagevec.h multiple times without
incurring errors due to duplicate definitions.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>