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/Spi.h

@@ -1,7 +1,7 @@
 #pragma once
 #include <cstdint>
 #include <cstddef>
-#include "SpiMaster.h"
+#include "drivers/SpiMaster.h"
 
 namespace Pinetime {
   namespace Drivers {