Fix ia64 build breakage in head.S

The "cleanup console_print()" patch in commit
353f6dd2

 introduced an "extern"
declaration into an assembly language file.  Remove it.

Signed-off-by: Anirban Sinha <asinha@zeugmasystems.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>