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/drivers/Watchdog.cpp

@@ -1,4 +1,4 @@
-#include "Watchdog.h"
+#include "drivers/Watchdog.h"
 #include <mdk/nrf.h>
 using namespace Pinetime::Drivers;