[tsan] Include unistd.h to declare gettid()

Summary: Try to fix https://lab.llvm.org/buildbot/#/builders/176/builds/14400.

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Differential Revision: https://phabricator.intern.facebook.com/D60250762

Author: Kai Luo

compiler-rt/test/tsan/debug_alloc_stack.cpp

@@ -6,6 +6,7 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #ifndef __APPLE__
 #include <sys/types.h>