Parse forward definiton of structs.

src/interfaces/ecpg/ChangeLog

src/interfaces/ecpg/preproc/pgc.l

src/interfaces/ecpg/preproc/preproc.y

src/interfaces/ecpg/preproc/type.c

src/interfaces/ecpg/preproc/type.h

src/interfaces/ecpg/preproc/variable.c