file-has-acl: suppress a warning from gcc -Wsuggest-attribute=const

file-has-acl: suppress a warning from gcc -Wsuggest-attribute=const

* lib/file-has-acl.c (file_has_acl): This function (for some #ifdefs)
would evoke a new gcc warning.  Given all of the #ifdefs, it is better
not even to try to add the attribute.  Instead, add a pragma to suppress
the suggestion/warning.