Enable symbolize-deadlock.test on Windows

Summary:
Remove unneeded #include of <unistd.h> from SymbolizeDeadlock.cpp to
enable compilation on Windows and enable symbolize-deadlock.test on
Windows.

Patch by Jonathan Metzman (@metzman).

Reviewers: Dor1s

Reviewed By: Dor1s

Subscribers: kcc, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D51685

llvm-svn: 341473