c9fe1da748
Fixes: b8b97209 stm32l4: basic uart/exti/led/clock demo
which introduced the l4 example, but left out the makefile.include file
and didn't add it to the root build.
Reported at: https://github.com/libopencm3/libopencm3-examples/pull/193/commits/2a7e4fac1438519d0a8ec83bfc1400795c562fe0