cogl/cogl-debug.h: Export and properly use _cogl_debug_instances

We need to also export the extern variables _cogl_debug_instances its
consumption in Cogl-Path, so that it can be properly referred to by
Cogl-Path and hence any other libraries that make use of them, such as

On Visual Studio DLL builds, this *must* also be marked with
__declspec(dllimport) when Cogl-Path is being built against Cogl so that
it can be referred to properly, otherwise one will be subject to crashes
as a result.
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