Hook up nm to check for leaking symbols in the static library

Hook up nm to check for leaking symbols in the static library

We ignore anything that starts with an underscore, anything that starts with
libevdev and main (since we test the test-static-link binary) and a couple of
gcov-related functions. This should catch any functions we accidentally
export.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>