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

(cherry picked from commit da36b5d244419fdc137e0cce50bf021e8c46c822)