compiler: Prefix headers for set_cheader_override() if includedir is given
compiler: Prefix headers for set_cheader_override() if includedir is given

We need to account for changes done by SourceFile.get_cinclude_filename()

https://bugzilla.gnome.org/show_bug.cgi?id=793299

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