compiler: Swap 'public domain' header for license

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: Felipe Franciosi <[email protected]>
Message-Id: <1474642971[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>