Fix FindEGL

Summary:
${EGL_INCLUDE_DIR} is the path up to 'egl.h' -- so the header is
${EGL_INCLUDE_DIR}/egl.h

The compile test on the other hand includes 'EGL/egl.h', so the path
that needs to be passed to the compile test is "${EGL_INCLUDE_DIR}/..".

Reviewers: #automotive, hausmann, #freebsd, apol

Reviewed By: #automotive, hausmann, apol

Subscribers: bcooksley, kde-frameworks-devel, kde-buildsystem

Tags: #frameworks, #build_system

Differential Revision: https://phabricator.kde.org/D19075