Add missing #include for linux.

<rdar://problem/25501013>

llvm-svn: 272445
diff --git a/lldb/source/Target/ThreadCollection.cpp b/lldb/source/Target/ThreadCollection.cpp
index cdccc4b..d8d622e 100644
--- a/lldb/source/Target/ThreadCollection.cpp
+++ b/lldb/source/Target/ThreadCollection.cpp
@@ -9,6 +9,7 @@
 #include <stdlib.h>
 
 #include <algorithm>
+#include <mutex>
 
 #include "lldb/Target/ThreadCollection.h"
 #include "lldb/Target/Thread.h"