Update includes to to be relative to src directory

Don't use relative imports like `../foo.h` as those depend on the
relative position of both files. Rather than that use imports relative
to the `src` directory, which explicitly is part of the include
directories.

src/components/firmwarevalidator/FirmwareValidator.cpp

@@ -1,4 +1,4 @@
-#include "FirmwareValidator.h"
+#include "components/firmwarevalidator/FirmwareValidator.h"
 
 #include <hal/nrf_rtc.h>
 #include "drivers/InternalFlash.h"