It was generating #include with the basename of every header file. That
assumes that every directory where there are headers are also included
into search path when compiling the .c file.
Change to use path relative to current subdir, which can be both in
build or source directory. That means that we assume that when the .c
file is compiled, the target has a include_directories pointing to the
directory where gnome.mkenum_simple() has been called, which is
generally '.' and added automatically.
Also fix type annotation to only allow str and File sources, other types
have never been working, it would require to iterate over custom target
outputs, etc.
Fixes: #7582
Xavier Claessens