fix windows build

add the GL include (back) to Base.props
use a similar technique to GLX.cpp (by Sonic) in WGL.cpp to get
wglSwapIntervalEXT without the WGLEW check

Conflicts:
	Source/Core/VideoBackends/OGL/OGL.vcxproj
	Source/Core/VideoBackends/OGL/OGL.vcxproj.filters
	Source/VSProps/Base.props

Source/Core/VideoBackends/OGL/GLExtensions/GLExtensions.cpp

@@ -5,7 +5,9 @@
 #include "GLExtensions.h"
 #include "Log.h"
 
+#if defined(__linux__)
 #include <dlfcn.h>
+#endif
 #include <unordered_map>
 
 // gl_1_2

Source/Core/VideoBackends/OGL/GLExtensions/gl_common.h

@@ -2,6 +2,9 @@
 // Licensed under GPLv2
 // Refer to the license.txt file included.
 
+#include <GL/gl.h>
+#include <GL/glext.h>
+
 #ifndef GLAPIENTRY
 #define GLAPIENTRY
 #endif