Update utils.h, fix missing #include <string> (#421)

Author: hzqst

src/utils/utils.h

@@ -21,6 +21,7 @@
 
 #include "playerslot.h"
 #include "utlvector.h"
+#include <string>
 
 class CServerSideClient;
 
@@ -31,4 +32,4 @@ CUtlVector<CServerSideClient*>* GetClientList();
 CServerSideClient* GetClientBySlot(CPlayerSlot slot);
 
 uint32 GetSoundEventHash(const char* pszSoundEventName);
-std::string StringToLower(std::string strValue);
+std::string StringToLower(std::string strValue);