Update submodule ecl to latest Fri Aug 31 21:30:56 UTC 2018

    - ecl in PX4/Firmware (95b83ba29715c067a30cb9501cab51264db502f1): https://github.com/PX4/ecl/commit/56b8bb08a16f0e86f7c7dfe458f0d2d3082e92e4
    - ecl current upstream: https://github.com/PX4/ecl/commit/3c74dc2ce1002db5b68a833fdd0aa964210e91f5
    - Changes: https://github.com/PX4/ecl/compare/56b8bb08a16f0e86f7c7dfe458f0d2d3082e92e4...3c74dc2ce1002db5b68a833fdd0aa964210e91f5

    3c74dc2 2018-08-30 Daniel Agar - ECL_Controller base needs virtual destructor
0d39072 2018-08-30 Daniel Agar - EKF add missing header guards
a6a1edb 2018-08-30 Kabir Mohammed - EKF : Fix type of flow data navigation variable (#499)
a53ad9c 2018-08-25 Paul Riseborough - EKF: Add missing optical flow ground motion protection

ecl @ 3c74dc2c

-Subproject commit 56b8bb08a16f0e86f7c7dfe458f0d2d3082e92e4
+Subproject commit 3c74dc2ce1002db5b68a833fdd0aa964210e91f5