compiler: Swap 'public domain' header for license

As discussed on the list [1], having a comment stating that this file
is "public domain" is arguably wrong and not legally binding. This patch
replaces that comment with a clear GPLv2+ license as proposed in [2].

[1] http://lists.nongnu.org/archive/html/qemu-devel/2016-09/msg06151.html
[2] http://lists.nongnu.org/archive/html/qemu-devel/2016-09/msg06217.html

Worth noting, compiler.h was originally created on 5c026320 by splitting
qemu-common.h. At the time, qemu-common.h was already GPLv2+.
Signed-off-by: NFelipe Franciosi <felipe@nutanix.com>
Message-Id: <1474642971-11866-1-git-send-email-felipe@nutanix.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

include/qemu/compiler.h

-/* public domain */
+/* compiler.h: macros to abstract away compiler specifics
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
 
 #ifndef COMPILER_H
 #define COMPILER_H