qga: Add multiple include guard to guest-agent-core.h
The guest-agent-core.h header was missing the usual guards
against multiple inclusion; add them.
(Spotted by lgtm.com's static analyzer.)
Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com> Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com> Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>