install the opengl legacy examples

while they are not built, their sources should be installed as long we
don't delete them completely.

Change-Id: I5e628e96cc9715520cb6e5aadb2cae61d1d03a4f
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

examples/opengl/opengl.pro

@@ -16,3 +16,6 @@ qtHaveModule(widgets) {
                 textures \
                 hellogles3
 }
+
+EXAMPLE_FILES += \
+    legacy