Sprinkle some #include <mutex> in files that use std::call_once.

llvm-svn: 234005
diff --git a/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp b/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
index 00cbb20..2ff5e13 100644
--- a/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
+++ b/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp
@@ -24,6 +24,7 @@
 // C++ Includes
 #include <algorithm>
 #include <map>
+#include <mutex>
 
 // Other libraries and framework includes