Add missing include

Fixes this build error on Linix:

memory_manager.h:134:58: error: ‘string’ in namespace ‘std’ does not
name a type
134 | bool add_block_to_memory_pool_by_name(const std::string& pool_name
    |                                                  ^~~~~~

Core/SoarKernel/src/shared/memory_manager.h

@@ -33,6 +33,7 @@
 #include "kernel.h"
 
 #include <unordered_map>
+#include <string>
 
 #ifndef _WIN32
 #include <stdlib.h> // malloc