package/libglfw: fix wayland build

Fix the following build failure raised since bump to version 3.3.6 in
commit 3cd9bb4f:

CMake Error at CMakeLists.txt:252 (find_package):
  Could not find a package configuration file provided by "ECM" with any of
  the following names:

    ECMConfig.cmake
    ecm-config.cmake

Fixes:
 - http://autobuild.buildroot.org/results/0d7420aff7392c294614fd12e65ac7cd57787e98



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>