hexagon: process: add internal prototype for do_work_pending()

Clang warns:

  arch/hexagon/kernel/process.c:155:5: warning: no previous prototype for function 'do_work_pending' [-Wmissing-prototypes]
    155 | int do_work_pending(struct pt_regs *regs, u32 thread_info_flags)
        |     ^
  arch/hexagon/kernel/process.c:155:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
    155 | int do_work_pending(struct pt_regs *regs, u32 thread_info_flags)
        | ^
        | static

This function is only referenced from assembly, so it does not technically
need a prototype.  Add one right above the definition anyways to clear up
the warning.

Link: https://lkml.kernel.org/r/20231130-hexagon-missing-prototypes-v1-14-5c34714afe9e@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>