ath5k: fix undefined 'THIS_MODULE'

When cross compiling ath5k for a Mips machine with kernel 3.2.14
the compilation fails with "/ath5k/ahb.c:231:12: error: 'THIS_MODULE' undeclared here (not in a function)"
Fix the build by including <linux/export.h>

Signed-off-by: Jonathan Bither <jonbither@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath5k/ahb.c

@@ -19,6 +19,7 @@
 #include <linux/nl80211.h>
 #include <linux/platform_device.h>
 #include <linux/etherdevice.h>
+#include <linux/export.h>
 #include <ar231x_platform.h>
 #include "ath5k.h"
 #include "debug.h"