proc_fs.h: move struct mm_struct forward-declaration

Move the forward-declaration of struct mm_struct a little way up
proc_fs.h.  This fixes a bunch of "'struct mm_struct' declared inside
parameter list" warnings with CONFIG_PROC_FS=n

Signed-off-by: Ben Nizette <bn@niasdigital.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>