hexagon: signal: switch to SYSCALL_DEFINE0 for sys_rt_sigreturn()

Clang warns:

  arch/hexagon/kernel/signal.c:223:16: warning: no previous prototype for function 'sys_rt_sigreturn' [-Wmissing-prototypes]
    223 | asmlinkage int sys_rt_sigreturn(void)
        |                ^
  arch/hexagon/kernel/signal.c:223:12: note: declare 'static' if the function is not intended to be used outside of this translation unit
    223 | asmlinkage int sys_rt_sigreturn(void)
        |            ^
        |            static
  1 warning generated.

Switch to the SYSCALL_DEFINE0() macro, which automatically declares a
prototype.

Link: https://lkml.kernel.org/r/20231130-hexagon-missing-prototypes-v1-11-5c34714afe9e@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Brian Cain <bcain@quicinc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>