powerpc/uapi: Fix sigcontext definition to use user_pt_regs

My recent patch to split pt_regs between user and kernel missed
the usage in struct sigcontext.

Because this is a user visible struct it should be using the user
visible definition, which when we're building for the kernel is called
struct user_pt_regs.

As far as I can see this hasn't actually caused a bug (yet), because
we don't use the sizeof() the sigcontext->regs anywhere. But we should
still fix it to avoid confusion and future bugs.

Fixes: 002af939 ("powerpc: Split user/kernel definitions of struct pt_regs")
Reported-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/uapi/asm/sigcontext.h

@@ -22,7 +22,11 @@ struct sigcontext {
 #endif
 	unsigned long	handler;
 	unsigned long	oldmask;
-	struct pt_regs	__user *regs;
+#ifdef __KERNEL__
+	struct user_pt_regs __user *regs;
+#else
+	struct pt_regs	*regs;
+#endif
 #ifdef __powerpc64__
 	elf_gregset_t	gp_regs;
 	elf_fpregset_t	fp_regs;