Adjust function definition in riscv's db_trace.c to avoid clang 15 warning

Adjust function definition in riscv's db_trace.c to avoid clang 15 warning

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

    sys/riscv/riscv/db_trace.c:56:23: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    db_md_list_watchpoints()
                          ^
                           void

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

MFC after:	3 days