winex11.drv: only declare the winediag debug channel if OpenGL is enabled

[Austin English]

Gets rid of:
opengl.c:45:36: warning: '__wine_dbch_winediag' defined but not used

when --without-opengl is used.

-- 
-Austin
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diff --git a/dlls/winex11.drv/opengl.c b/dlls/winex11.drv/opengl.c
index 78a717d..0d4d822 100644
--- a/dlls/winex11.drv/opengl.c
+++ b/dlls/winex11.drv/opengl.c
@@ -42,10 +42,11 @@
 #include "wine/debug.h"
 
 WINE_DEFAULT_DEBUG_CHANNEL(wgl);
-WINE_DECLARE_DEBUG_CHANNEL(winediag);
 
 #ifdef SONAME_LIBGL
 
+WINE_DECLARE_DEBUG_CHANNEL(winediag);
+
 #undef APIENTRY
 #undef CALLBACK
 #undef WINAPI