Adjust function definition in acpi_timer.c to avoid clang 15 warnings

With clang 15, the following -Werror warning is produced:

sys/dev/acpica/acpi_timer.c:402:16: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
acpi_timer_test()
               ^
                void

This is because acpi_timer_test() is declared with a (void) argument
list, but defined with an empty argument list. Make the definition match
the declaration.

MFC after: 3 days

(cherry picked from commit c45bc02560b0f632f8e085679512a588bfcb3384)