Add missing #include to fix libc++ Linux build.

llvm-svn: 232925
diff --git a/lldb/source/Plugins/Process/Linux/NativeThreadLinux.h b/lldb/source/Plugins/Process/Linux/NativeThreadLinux.h
index c77dcbe..7f563d8 100644
--- a/lldb/source/Plugins/Process/Linux/NativeThreadLinux.h
+++ b/lldb/source/Plugins/Process/Linux/NativeThreadLinux.h
@@ -14,6 +14,7 @@
 #include "lldb/Host/common/NativeThreadProtocol.h"
 
 #include <map>
+#include <string>
 
 namespace lldb_private
 {