sh: ftrace: Fix missing prototypes

sh: ftrace: Fix missing prototypes

arch/sh/kernel/ftrace.c:130:6: warning: no previous prototype for ‘arch_ftrace_nmi_enter’ [-Wmissing-prototypes]
arch/sh/kernel/ftrace.c:140:6: warning: no previous prototype for ‘arch_ftrace_nmi_exit’ [-Wmissing-prototypes]
arch/sh/kernel/ftrace.c:316:6: warning: no previous prototype for ‘prepare_ftrace_return’ [-Wmissing-prototypes]

Fix this by moving existing forward declarations to <asm/ftrace.h>, and
adding the missing forward declaration for prepare_ftrace_return().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/910c8846a025e1c3b744a83ddf8e2816a3c5569d.1709326528.git.geert+renesas@glider.be
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>