[AIX][NFC] Enable response-file-errs.c

The test was fixed by commit 678f19f.

Author: jakeegan

clang/test/Driver/response-file-errs.c

@@ -1,6 +1,3 @@
-// AIX reacts on opening directory differently than other systems.
-// XFAIL: system-aix
-
 // If response file does not exist, '@file; directive remains unexpanded in
 // command line.
 //