core: #include <gmodule.h> for G_MODULE_EXPORT

When some configure-time options aren't used, <gmodule.h> is not
included.  Fix that.

src/devices/atm/nm-atm-manager.c

@@ -22,6 +22,7 @@
 
 #include <string.h>
 #include <gudev/gudev.h>
+#include <gmodule.h>
 
 #include "nm-atm-manager.h"
 #include "nm-device-adsl.h"

src/devices/bluetooth/nm-bluez-manager.c

@@ -21,6 +21,7 @@
 #include <signal.h>
 #include <string.h>
 #include <stdlib.h>
+#include <gmodule.h>
 #include <gio/gio.h>
 
 #include "nm-logging.h"

src/devices/wwan/nm-wwan-factory.c

@@ -19,6 +19,8 @@
  */
 
 #include <string.h>
+#include <gmodule.h>
+
 #include "config.h"
 #include "nm-device-factory.h"
 #include "nm-wwan-factory.h"