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

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

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>