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

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

sys/cddl/dev/dtrace/riscv/dtrace_subr.c:165:17: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
dtrace_gethrtime()
                ^
                 void

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

MFC after: 3 days