resouce file in tar break some automated build

[cbertelli]

I was investigating an issue with macbrew while I found the file ._pngquant_opts.c that caused a warning in tar.

-rw-r--r--  0 501    20        176 22 Nov 00:59 pngquant-2.17.0/._pngquant_opts.c
tar: Ignoring malformed pax extended attribute
-rw-r--r--  0 501    20       5812 22 Nov 00:59 pngquant-2.17.0/pngquant_opts.c

There are workarounds, but it's better to get rid of it.