KVM: selftests: Remove superfluous define from vmx.c

The code in vmx.c does not use "program_invocation_name", so there
is no need to "#define _GNU_SOURCE" here.
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/lib/x86_64/vmx.c

@@ -5,8 +5,6 @@
  * Copyright (C) 2018, Google LLC.
  */
 
-#define _GNU_SOURCE /* for program_invocation_name */
-
 #include "test_util.h"
 #include "kvm_util.h"
 #include "processor.h"