- add functions prototypes for some global functions to header files

- remove unneeded "extern"s from some function prototypes

You might note that this patch results in a new warning - that's due to the
fact that with a proper prototype gcc is able to discover a broken
work_struct conversion.
Signed-off-by: NAdrian Bunk <bunk@stusta.de>
Cc: Karsten Keil <kkeil@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>