Adjust function definition in arm64's db_trace.c to avoid clang 15 warning

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

sys/arm64/arm64/db_trace.c:53:23: 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 940740891516c31a993cb0d267bd19f2d07ede49)