Fixes the build error caused by -Werror on gcc 3.x compilers:

arch/sh/kernel/signal_32.c: In function `sys_sigaction':
arch/sh/kernel/signal_32.c:66: warning: initialization discards qualifiers from pointer target type
arch/sh/kernel/signal_32.c:67: warning: initialization discards qualifiers from pointer target type
arch/sh/kernel/signal_32.c:69: warning: initialization discards qualifiers from pointer target type
arch/sh/kernel/signal_32.c:70: warning: initialization discards qualifiers from pointer target type

The mismatch in question was introduced by commit-id
9c5a4eec79b3eb8876d2e7fddfa1e040a7650e55.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>