[TF2] Fix the Air Strike's kill counter incrementing due to reflect kills

[Tkain]

Description

In TF2, the Air Strike's kill count (and clip size) increases for each kill its wielder gets using it. However, these effects also apply to an Air Strike user if an enemy Pyro gets a kill from reflecting the user's Air Strike rocket. This is likely unintended, since it essentially rewards a player for a kill they didn't get. (It's also listed as a bug on the TF2 Wiki.)

This occurs because CTFRocketLauncher_AirStrike::OnPlayerKill increases the kill count of the user of an Air Strike whenever one of its rockets kills a player even though the Air Strike user may not be responsible for the kill (as with a reflect kill). This PR fixes this by having the function increase the kill count of the actual killer rather than the Air Strike's user.