powerpc: Add a missing include header

The header file <asm/switch_to.h> was missing from the includes. Fix the
following warning, treated as error with W=1:

  arch/powerpc/kernel/vecemu.c:260:5: error: no previous prototype for ‘emulate_altivec’ [-Werror=missing-prototypes]
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/vecemu.c

@@ -8,6 +8,7 @@
 #include <linux/sched.h>
 #include <asm/ptrace.h>
 #include <asm/processor.h>
+#include <asm/switch_to.h>
 #include <linux/uaccess.h>
 
 /* Functions in vector.S */