egl: Fix CMake traversal into egl/spec dir

CMake should traverse into subdirectories independent of the current
API. That is, the generic CMakeLists.txt should call
add_subdirectory(xxx), not the api-specific CMakeLists.$api.txt.  For
precedent, see tests/spec/CMakeLists.txt.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>