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 <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Max Filippov <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Max Filippov <[email protected]>

Author: rddunlap

arch/xtensa/kernel/stacktrace.c

@@ -12,6 +12,7 @@
 #include <linux/sched.h>
 #include <linux/stacktrace.h>
 
+#include <asm/ftrace.h>
 #include <asm/stacktrace.h>
 #include <asm/traps.h>
 #include <linux/uaccess.h>