IBMASM: don't use extern in function declarations

We normally don't use extern in function declarations located in header files.
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Cc: Vernon Mauery <vernux@us.ibm.com>
Cc: Max Asbock <masbock@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>