xtensa: stacktrace: include <asm/ftrace.h> for prototype

Use <asm/ftrace.h> to prevent a build warning:

arch/xtensa/kernel/stacktrace.c:263:15: warning: no previous prototype for 'return_address' [-Wmissing-prototypes]
  263 | unsigned long return_address(unsigned level)

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20230920052139.10570-8-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>