[PATCH] lyxpreview_tools.py: fix a regular expression

[Neven Sajko]

Actually, do something like this (no "using distributivity" like in my
first message, to get a minimal git diff)

def_re = re.compile(r"(\newcommandx|\renewcommandx|\global\long\def)(\[a-zA-Z]+)(.+)")

Regards,
Neven Sajko