BUGFIX: Add conditional compilation to support S110 only builds

S110 builds contian a conditional compilation option that compiles out
GattClient functionality, thereby saving several hundred bytes of unecessary
memory overhead.

source/btle/btle.cpp

@@ -191,9 +191,11 @@ static void btle_handler(ble_evt_t *p_ble_evt)
             }
 
             // Close all pending discoveries for this connection
+#if !defined(TARGET_MCU_NRF51_16K_S110) && !defined(TARGET_MCU_NRF51_32K_S110)
             nRF5xGattClient& gattClient = ble.getGattClient();
             gattClient.characteristicDescriptorDiscoverer().terminate(handle, BLE_ERROR_INVALID_STATE);
             gattClient.discovery().terminate(handle);
+#endif
 
             gap.processDisconnectionEvent(handle, reason);
             break;