fetchmail: adjust zlib handling

fetchmail itself does not need zlib, there is no reference to it in its
source, so the "select BR2_PACKAGE_ZLIB" is not necessary. Moreover,
"zlib" was not added to FETCHMAIL_DEPENDENCIES (but was anyway in the
dependency chain through openssl).

In addition, LIBS="-lz" is only needed in static linking configurations,
to help fetchmail's configure script find OpenSSL (it doesn't use
pkg-config unfortunately).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>