Add missing const's

[VirxEC]

Added const to the following functions:

• Arena::GetTickRate
• Ball::GetRadiusBullet
• Ball::GetRadius
• BallHitInfo::Serialize
• BoostPadState::Serialize
• SuspensionCollisionGrid::GetCellMin
• SuspensionCollisionGrid::GetCellIndicesFromPos
• SuspensionCollisionGrid::GetCellSize

Also added the build/ directory to .gitignore for personal reasons

[ZealanL]

I think you may have accidentally replaced Arena.h with Arena.cpp

[VirxEC]

LOL whoops