Fix m_zoneLayerOverrides data race in pad.h

From Coverity issue 542093

pcbnew/pad.cpp

@@ -201,6 +201,8 @@ void PAD::ClearZoneLayerOverrides()
 
 const ZONE_LAYER_OVERRIDE& PAD::GetZoneLayerOverride( PCB_LAYER_ID aLayer ) const
 {
+    std::unique_lock<std::mutex> cacheLock( m_zoneLayerOverridesMutex );
+
     static const ZONE_LAYER_OVERRIDE defaultOverride = ZLO_NONE;
     auto it = m_zoneLayerOverrides.find( aLayer );
     return it != m_zoneLayerOverrides.end() ? it->second : defaultOverride;

pcbnew/pad.h

@@ -955,6 +955,6 @@ private:
 
     int         m_lengthPadToDie;   // Length net from pad to die, inside the package
 
-    std::mutex                                  m_zoneLayerOverridesMutex;
+    mutable std::mutex                          m_zoneLayerOverridesMutex;
     std::map<PCB_LAYER_ID, ZONE_LAYER_OVERRIDE> m_zoneLayerOverrides;
 };