Fix: Enabling a precompiled header and multiple architectures results in a malformed Visual Studio project

[ryaanahmed-bl]

A project with a precompiled header and multiple platforms selected for a single configuration would result in multiple ClCompile entries in the Visual Studio project file, which is an invalid file state and would not open in Visual Studio.

Projucer now correctly emits a single ClCompile entry for the PCH, including entries for all target platforms.

Fixes #1519