Fix some compilation errors, add FixedWing files to CMake

[rajat2004]

This is just a small beginning, to fix the compilation error (utterly unhelpful and useless message)-

FixedWingPawn.h(21) : Error: Missing '*' in Expected a pointer type

Problem was that a UPROPERTY was specified (declared?) in L19, without any variable below it, commenting that out now gives normal compilation errors which can be slowly fixed.
Others are just some #include errors, and a currently-commented line to add the FixedWing files in CMake (used on Linux). Can be uncommented after fixing all the problems

Have tried to fix a few errors, still lots of them are left, might be best if you have a look and decide which APIs to keep etc.
Later on when things are working, an extensive review of the APIs will mostly be required to decide the final ones

[AOS55]

Thanks a lot for this, really helpful, Ill start working through them.

[rajat2004]

Glad to help!