Log Message:
libXdmcp: apply upstream version of SunOS compilation fix

Patch to match what the next release will do here, in part so we don't
run the risk of merging two versions of handling the same issue on next
update. Addresses PR pkg/54628. (Compile tested on Fedora Linux and
OmniOS, but as neither of these are affected by the issue to begin
with, and not having an environment where I can reproduce it, I can't
verify the fix myself.)