GUACAMOLE-1259: Include missing config.h header for sake of conditional Stream_Free().

The changes introduced by GUACAMOLE-1181 (commit 2c86e20) were made
conditional as older versions of FreeRDP will automatically free the
wStream, resulting in a double-free if we attempt to do so ourselves.

The macro controlling that conditional code is defined within config.h,
which is missing here. Without that macro, the call to Stream_Free()
always occurs, and we get a double-free with older FreeRDP.

src/protocols/rdp/plugins/guac-common-svc/guac-common-svc.c

@@ -17,6 +17,7 @@
  * under the License.
  */
 
+#include "config.h"
 #include "channels/common-svc.h"
 
 #include <freerdp/svc.h>