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