ipq-wifi: add missing dependency on TARGET_ath79

ipq-wifi: add missing dependency on TARGET_ath79

A previous commit backported the ipq-wifi update to fix support for the
TP-Link Archer C6 v2 by adding the device to the package. However, it
missed adding the required TARGET_ath79 dependency, causing the
ipq-tplink_archer-c6-v2 build to fail.

The dependency was previously added in commit 4990ce613b7d ("ipq-wifi:
update to 2024-02-17") when several ath79 devices were introduced.
However, since this backport only fixes support for the Archer C6 v2, it
is not feasible to backport all related changes. Therefore, this commit
adds only the missing dependency to resolve the build issue without
pulling in unrelated updates.

Fixes: 0c43acc349a7 ("ipq-wifi: update to Git HEAD (2025-05-30)")
Signed-off-by: Nick Hainke <vincent@systemli.org>
Link: https://github.com/openwrt/openwrt/pull/19120
Signed-off-by: Robert Marko <robimarko@gmail.com>