simplify windows dllexport again
simplify windows dllexport again

declspec seems to be the "standard" way on Windows.

Adding the __attribute__ version didn't make a difference.
I was just testing wrong (with nm, like on Linux).

The correct test was:
ojbdump.exe -p libdiscid.dll | grep -A 30 "Ordinal/Name Pointer"

metabrainz/libdiscid,metabrainz/libdiscid,phw/libdiscid,phw/libdiscid