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>

qga/guest-agent-core.h

diff --git a/qga/guest-agent-core.h b/qga/guest-agent-core.h
index 6f4d214..60eae16 100644
--- a/qga/guest-agent-core.h
+++ b/qga/guest-agent-core.h
@@ -10,6 +10,9 @@
  * 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 GUEST_AGENT_CORE_H
+#define GUEST_AGENT_CORE_H
+
 #include "qapi/qmp/dispatch.h"
 #include "qemu-common.h"
 #include "qga-qapi-types.h"
@@ -46,3 +49,5 @@
 #ifndef _WIN32
 void reopen_fd_to_null(int fd);
 #endif
+
+#endif /* GUEST_AGENT_CORE_H */