build: avoid compiler warning

build: avoid compiler warning

According to GCC, ATTRIBUTE_UNUSED means that an attribute _might_
be unused, not _must_ be unused.  Therefore, it is easier to
blindly mark a variable, than to try and do preprocessor limiting
of when we know it is unused.

* src/remote/remote_driver.c (remoteAuthenticate): Mark attribute
as potentially unused.
Reported by Gustovo Morozowski.