wined3d: Remove unneeded "extern" from function definition

[Andrew Talbot]

Changelog:
    wined3d: Remove unneeded "extern" from function definition.

diff --git a/dlls/wined3d/surface.c b/dlls/wined3d/surface.c
index 7a1a355..684690b 100644
--- a/dlls/wined3d/surface.c
+++ b/dlls/wined3d/surface.c
@@ -2489,7 +2489,7 @@ HRESULT WINAPI IWineD3DSurfaceImpl_SaveSnapshot(IWineD3DSurface *iface, const ch
 /**
  *   Slightly inefficient way to handle multiple dirty rects but it works :)
  */
-extern HRESULT WINAPI IWineD3DSurfaceImpl_AddDirtyRect(IWineD3DSurface *iface, CONST RECT* pDirtyRect) {
+HRESULT WINAPI IWineD3DSurfaceImpl_AddDirtyRect(IWineD3DSurface *iface, CONST RECT* pDirtyRect) {
     IWineD3DSurfaceImpl *This = (IWineD3DSurfaceImpl *)iface;
     IWineD3DBaseTexture *baseTexture = NULL;