Codechange: Add 'const' to static variables, which are only initialised once.

src/screenshot_pcx.cpp

@@ -153,4 +153,4 @@ public:
 	}
 };
 
-static ScreenshotProvider_Pcx s_screenshot_provider_pcx;
+static const ScreenshotProvider_Pcx s_screenshot_provider_pcx;