Add header so cellular_power tests compile and run

Author: felser

features/cellular/TESTS/api/cellular_power/main.cpp

@@ -37,6 +37,7 @@
 
 #include "mbed.h"
 
+#include "AT_CellularPower.h"
 #include "CellularDevice.h"
 #include "../../cellular_tests_common.h"
 #include CELLULAR_STRINGIFY(CELLULAR_DEVICE.h)