girparser: Don't guess length of xml header, iterate forward to <repository>
girparser: Don't guess length of xml header, iterate forward to <repository>

If e.g. the xml header is missing, it resulted in errors like:

    error: expected start element of `repository'
    error: unsupported GIR version (null) (supported: 1.2)

frida/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala