pb.h should be an include from the local directory, therefore #include "ph.h" and not #include <pb.h>.

Compile will fail if this library is not being included from the Arduino global path, but rather from the local directory. For example if the directory structure looks like this:

example/
- example.ino
- src/
-- arduino-device-lib/

and inside example.ino we include the library as follows:

#include "src/arduino-device-lib/src/TheThingsNetwork.h"

After this change the compile is successful.

Note:
If the library is present in both the Arduino libraries directory as well as the local directory, compilation might fail due to duplicate definitions. Keep this in mind during testing.