sh: boot: Add proper forward declarations

sh: boot: Add proper forward declarations

arch/sh/boot/compressed/misc.c:110:6: warning: no previous prototype for ‘ftrace_stub’ [-Wmissing-prototypes]
arch/sh/boot/compressed/misc.c:113:6: warning: no previous prototype for ‘arch_ftrace_ops_list_func’ [-Wmissing-prototypes]
arch/sh/boot/compressed/misc.c:123:6: warning: no previous prototype for ‘decompress_kernel’ [-Wmissing-prototypes]

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/b7ea770a3bf26fb2a5f59f4bb83072b2526f7134.1713959841.git.geert+renesas@glider.be
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>