include symbols from c sources files on exported header/vapi/gir file
currently, if you compile your code with --library
and include .c and .h files the generated header/vapi/gir file don't include the declared functions/classes of the c sources only the vala ones.
it can be useful if valac added the classes and functions declared in theses files since, for now, you will need to edit manually the generated header/vapi/gir file to include the functions/classes declared on theses file.