Adjust dtnfsclient_unload() definition to avoid clang 15 warning

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

sys/fs/nfsclient/nfs_clkdtrace.c:544:19: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
dtnfsclient_unload()
                  ^
                   void

This is because dtnfsclient_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