fails to compile with nanopb-0.4.8

[blshkv]

nanopb has change the default location for headers:

-$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/nanopb>

it also provides *.cmake helpers to find it automatically.

However, qFlipper does not use cmake build system and unable to detect that change.
Obviously, it would fail to find it with the following error:

"fatal error: pb.h: No such file or directory"

The solution: add the new path to INCLUDEPATH variable (I guess)