Adjust dtmalloc_unload() definition to avoid clang 15 warning

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

sys/cddl/dev/dtmalloc/dtmalloc.c:177:16: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
dtmalloc_unload()
               ^
                void

This is because dtmalloc_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 a0c55bac79ef684db5f3645fdc1e0fd8da827985)