Annotate unused param in evdi_gem_mmap

[richgieg]

Fixes the build issue mentioned in @WallaceDog's second comment in #573 (#573 (comment))

When running make from within the module directory there was no issue (which is how I usually build the module so I didn't see this before), but when running make from within the project's root directory the -Wextra caused the build to fail on this unused dev param.

[richgieg]

Just wanted to add that the build succeeds without this fix on EL 10.2, but the __always_unused annotation is needed for the build to succeed on EL 9.8. Perhaps on GCC 14.3.1 the -Wextra doesn't check for unused params but on GCC 11.5.0 it does?

richard@rocky10:~/evdi$ gcc --version
gcc (GCC) 14.3.1 20251022 (Red Hat 14.3.1-4)

[richard@rocky9 ~]$ gcc --version
gcc (GCC) 11.5.0 20240719 (Red Hat 11.5.0-14)