sh: Fix declaration of __kernel_sigreturn and __kernel_rt_sigreturn

GCC 4.5.0 complains about the declaration of variables
__kernel_sigreturn and __kernel_rt_sigreturn because they have type
void.  Correctly declare these symbols as functions to fix the
following error,

arch/sh/kernel/signal_32.c: In function 'setup_frame':
arch/sh/kernel/signal_32.c:368:14: error: taking address of expression of type 'void'
arch/sh/kernel/signal_32.c: In function 'setup_rt_frame':
arch/sh/kernel/signal_32.c:452:14: error: taking address of expression of type 'void'
make[1]: *** [arch/sh/kernel/signal_32.o] Error 1
make: *** [arch/sh/kernel] Error 2

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>