sh64: add missing #include <asm/fpu.h>'s

This patch fixes some compile errors due to missing save_fpu()
prototypes on sh64 caused by
commit 9bbafce2
(sh: Fix occasional FPU register corruption under preempt).

Signed-off-by: Adrian Bunk <adrian.bunk@movial.fi>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>