Add header needed to use the SWAP_PER_DRAW debug facility on windows.



git-svn-id: http://skia.googlecode.com/svn/trunk@4327 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/gl/GrGpuGL.cpp b/src/gpu/gl/GrGpuGL.cpp
index 5b03500..2662f8e 100644
--- a/src/gpu/gl/GrGpuGL.cpp
+++ b/src/gpu/gl/GrGpuGL.cpp
@@ -1658,6 +1658,7 @@
     #if GR_MAC_BUILD
         #include <AGL/agl.h>
     #elif GR_WIN32_BUILD
+        #include <gl/GL.h>
         void SwapBuf() {
             DWORD procID = GetCurrentProcessId();
             HWND hwnd = GetTopWindow(GetDesktopWindow());