powerpc/process: Add missing include of stacktrace.h

As spotted by sparse:

  arch/powerpc/kernel/process.c:1302:6: warning: symbol 'show_user_instructions' was not declared. Should it be static?

Fixes: 88b0fe1757359 ("powerpc: Add show_user_instructions()")
Reviewed-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Split out of larger patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/process.c

@@ -65,6 +65,7 @@
 #include <asm/livepatch.h>
 #include <asm/cpu_has_feature.h>
 #include <asm/asm-prototypes.h>
+#include <asm/stacktrace.h>
 
 #include <linux/kprobes.h>
 #include <linux/kdebug.h>