qga/channel-posix.c: Include headers it needs

Include:
 - <errno.h> for errno
 - <unistd.h> & <fcntl.h> for fcntl()
 - <stdlib.h> for exit()
 - "osdep.h" for qemu_open()

Some of those headers were probably being included by accident because
some other headers were including qemu-common.h, but those headers
should eventually stop including qemu-common.h.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

qga/channel-posix.c

@@ -1,5 +1,10 @@
 #include <glib.h>
 #include <termios.h>
+#include <errno.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include "osdep.h"
 #include "qemu_socket.h"
 #include "qga/channel.h"