This makes it possible to compile the Raspberry Pi gateway on
Raspbian 9 (stretch), even without adding
--extra-cxxflags="-D__STDC_FORMAT_MACROS"
(as was suggested in
https://github.com/mysensors/MySensors/issues/955 but was
insufficient for remediating this problem)
The compile problem was inadvertently introduced in
2d5404de9.
This change fixes the problem discussed in
https://github.com/mysensors/MySensors/issues/980