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

sys/cddl/dev/dtrace/riscv/dtrace_subr.c

@@ -162,7 +162,7 @@ dtrace_sync(void)
  * Returns nanoseconds since boot.
  */
 uint64_t
-dtrace_gethrtime()
+dtrace_gethrtime(void)
 {
 	struct timespec curtime;