webkitgtk: Allow building with BR2_PACKAGE_WEBKITGTK_MULTIMEDIA disabled

Explicitly pass "-DENABLE_MEDIA_STREAM=OFF" to CMake, to workaround a
missing feature dependency in the WebKitGTK+ build files.

Related upstream bug: https://bugs.webkit.org/show_bug.cgi?id=174940



Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>