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>

arch/sh/kernel/signal_32.c

@@ -332,8 +332,8 @@ get_sigframe(struct k_sigaction *ka, unsigned long sp, size_t frame_size)
 
 /* These symbols are defined with the addresses in the vsyscall page.
    See vsyscall-trapa.S.  */
-extern void __user __kernel_sigreturn;
-extern void __user __kernel_rt_sigreturn;
+extern void __kernel_sigreturn(void);
+extern void __kernel_rt_sigreturn(void);
 
 static int setup_frame(int sig, struct k_sigaction *ka,
 			sigset_t *set, struct pt_regs *regs)