obs-ffmpeg: avoid w32-pthreads for mingw-w64

mingw-w64 provides its own pthreads library

plugins/obs-ffmpeg/CMakeLists.txt

@@ -1,6 +1,6 @@
 project(obs-ffmpeg)
 
-if(WIN32)
+if(MSVC)
 	set(obs-ffmpeg_PLATFORM_DEPS
 		w32-pthreads)
 endif()