flatpak_generator: Fix regex
    
    Original regex just matched multiline C-like comments and did
    mistakes when sometimes matched filesystem
    wildcard "*" (eg. in /usr/lib/*.so) as a start of the comment.
    
    New regex forbids nested comments (which are forbidden in C as well)
    which also solves the problem above (no trailing */ found -> no match).
    
    There still are some very unprobable situations when this
    (as well as the old one) regex fails (like /usr/*/app-version*/) though.