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>