qga/channel-posix.c: Explicitly include string.h

Explicitly include string.h to avoid warnings under MacOS X/clang
about implicit declarations of strerror() and strlen().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

qga/channel-posix.c

@@ -4,6 +4,7 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <stdlib.h>
+#include <string.h>
 #include "qemu/osdep.h"
 #include "qemu/sockets.h"
 #include "qga/channel.h"