Fix bug #8552

Fix bug #8552

tex2lyx skips LyX preamble code only if it thinks that the file was created
by LyX (i.e. special comments are found). I don't like that, but this is
how it works and therefore the special comment neds to be added if the
theorem commands are to be skipped. This header makes also the temporary
setting of in_lyx_preamble obsolete, which caused skipping of user preamble
commands, which were not re-added by LyX.