x86/ftrace: Move prepare_ftrace_return prototype to header

On 32-bit builds, the prepare_ftrace_return() function only has a global
definition, but no prototype before it, which causes a warning:

arch/x86/kernel/ftrace.c:625:6: warning: no previous prototype for ‘prepare_ftrace_return’ [-Wmissing-prototypes]
  625 | void prepare_ftrace_return(unsigned long ip, unsigned long *parent,

Move the prototype that is already needed for some configurations into
a header file where it can be seen unconditionally.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://lore.kernel.org/all/20230516193549.544673-2-arnd%40kernel.org

arch/x86/include/asm/ftrace.h

@@ -106,6 +106,9 @@ struct dyn_arch_ftrace {
 
 #ifndef __ASSEMBLY__
 
+void prepare_ftrace_return(unsigned long ip, unsigned long *parent,
+			   unsigned long frame_pointer);
+
 #if defined(CONFIG_FUNCTION_TRACER) && defined(CONFIG_DYNAMIC_FTRACE)
 extern void set_ftrace_ops_ro(void);
 #else

arch/x86/kernel/ftrace.c

@@ -525,9 +525,6 @@ static void *addr_from_call(void *ptr)
 	return ptr + CALL_INSN_SIZE + call.disp;
 }
 
-void prepare_ftrace_return(unsigned long ip, unsigned long *parent,
-			   unsigned long frame_pointer);
-
 /*
  * If the ops->trampoline was not allocated, then it probably
  * has a static trampoline func, or is the ftrace caller itself.