csky: fix arch_jump_label_transform_static override

The arch_jump_label_transform_static() function in csky was originally meant to
override the generic __weak function, but that got changed to an #ifndef check.

This showed up as a missing-prototype warning:
arch/csky/kernel/jump_label.c:43:6: error: no previous prototype for 'arch_jump_label_transform_static' [-Werror=missing-prototypes]

Change the method to use the new method of having a #define and a prototype
for the global function.

Fixes: 7e6b9db2 ("jump_label: make initial NOP patching the special case")
Fixes: 4e8bb4ba ("csky: Add jump-label implementation")
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>