build: Add enumeration files to BUILT_SOURCES
build: Add enumeration files to BUILT_SOURCES

These files are generated, but since dependencies are a by-product of
compilation, Automake won't be able to generate the dependencies before
the files that include the headers are built.

For more information:

https://www.gnu.org/software/automake/manual/html_node/Sources.html

pwithnall/libgdata,pwithnall/libgdata,GNOME/libgdata