Adjust dtrace_unload() definition to avoid clang 15 warning

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

In file included from sys/cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c:18440:
sys/cddl/dev/dtrace/dtrace_unload.c:26:14: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
dtrace_unload()
             ^
              void

This is because dtrace_unload() 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 6f5f44562a72e739a39057a4661a56498e4c79b5)