shared_bindings_matrix: Use a trivial target
shared_bindings_matrix: Use a trivial target

Sommersoft noticed that without submodules (as RTD currently does),
the "make -qp" failed due to trying to find files within submodules.

The "print-VAR" target doesn't have any dependencies, so it sidesteps
the problem.

If we move ulab's pyi files into the submodule, though, we'll need to
fetch submodules anyway during doc building.

adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython