package/mutt: add zlib optional dependency

zlib is an optional dependency which is enabled by default since version
1.14.1 and
https://gitlab.com/muttmua/mutt/-/commit/136ae0add512f21bc418f9e31a2f1b970ad1a490



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>