win32: Fix stability typo in libgtksourceview-3.0-1.dll's DllMain

Release of global resources was accidently performed on any thread exit instead of process exit/dll unload. Premature releasing caused different stability problems.

This fixes meld#192 (closed) Same issue at msys2 github - https://github.com/Alexpux/MINGW-packages/issues/3510

Problem is specific for gnome-3-24 branch (master has this code removed), but makes gtksourceview 3-24 mostly unusable on windows.