Fix use of undeclared core debug category symbols

Fix use of undeclared core debug category symbols

libgstreamer currently exports some debug category
symbols GST_CAT_*, but those are not declared in any
public headers.

Some plugins and libgstvideo just use GST_DEBUG_CATEGORY_EXTERN()
to declare and use those, but that's just not right at
all, and it won't work on Windows with MSVC. Instead look
up the categories via the API.