package_rpm: use zstd's default compression level

package_rpm: use zstd's default compression level

zstd uses 3 by default, while 19 is the highest and slowest.
It's not clear why 19 was picked to begin with, possibly
I copy-pasted it from rpm's examples without thinking:
https://git.yoctoproject.org/poky/commit/?h=master-next&id=4a4d5f78a6962dda5f63e9891825c80a8a87bf66

This brings significant speedups in rpm's compression step:
for example compressing webkitgtk takes 11s instead of 36s.

The rpm size increases from 175648k to 234860k. I think it's
a worthy default tradeoff.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Steve Sakoman <steve@sakoman.com>