scripts/kernel-doc: recognize __meminit

Fix scripts/kernel-doc to recognize __meminit in a function prototype
and to strip it, as done with many other attributes.

Fixes this warning:

  Warning(..//mm/page_alloc.c:2973): cannot understand function prototype: 'void * __meminit alloc_pages_exact_nid(int nid, size_t size, gfp_t gfp_mask) '

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>