[Threading][Windows] Fix missing close brace.
[Threading][Windows] Fix missing close brace.

Apparently I missed a close brace from the end of Win32.h.

rdar://90776105

benlangmuir/swift,JGiola/swift,JGiola/swift,roambotics/swift,roambotics/swift,apple/swift,apple/swift,glessard/swift,apple/swift,glessard/swift,JGiola/swift,JGiola/swift,glessard/swift,JGiola/swift,apple/swift,roambotics/swift,benlangmuir/swift,glessard/swift,glessard/swift,benlangmuir/swift,benlangmuir/swift,roambotics/swift,roambotics/swift,benlangmuir/swift,apple/swift,JGiola/swift,glessard/swift,roambotics/swift,apple/swift,benlangmuir/swift