Fix linux/clang build with UE 4.5 github sources

While testing my own Git plugin, here is my small contribution to you.

First of all, it compiles flawlessly with UE4.5 on MSVC 2013 under Windows 7.
Then, with those fixes it also compiles with UE4.5 GitHub sources on clang 3.3 under Ubuntu 14.04 64 bits

 - #if PLAFTORM_WINDOWS instead of #ifdef PLAFTORM_WINDOWS
 - #if SOURCE_CONTROL_WITH_SLATE arround MakeSettingsWidget() like in all other source control providers