Include CStrInternStatic where necessary only.

This makes incremental recompilation faster when changing static
interned strings.

Differential Revision: https://code.wildfiregames.com/D3975
This was SVN commit r25457.

source/graphics/Material.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2012 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -18,6 +18,7 @@
 #include "precompiled.h"
 
 #include "Material.h"
+#include "ps/CStrInternStatic.h"
 
 static CColor BrokenColor(0.3f, 0.3f, 0.3f, 1.0f);