ath5k: use more idiomatic tracing include style

Pretty much everywhere that uses a trace definition
header that's not in include/trace/events/ uses the
make system for the include path rather than putting
it into the sources, so do that in ath5k as well.

This came up during backporting work (where this is
required), but since all other drivers do it this
way upstream it seemed applicable.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>