Noticed missing include as a build failure on gcc-11:
```
[ 15%] Building CXX object Source/Core/Common/CMakeFiles/common.dir/Config/Config.cpp.o
Source/Core/Common/Config/Config.cpp:23:24:
error: 'unique_lock' in namespace 'std' does not name a template type
23 | using WriteLock = std::unique_lock<std::shared_mutex>;
| ^~~~~~~~~~~
Source/Core/Common/Config/Config.cpp:11:1:
note: 'std::unique_lock' is defined in header '<mutex>';
did you forget to '#include <mutex>'?
```
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
447de580ab