package/rtty: fix build with mbedtls but without zlib

zlib is not mandatory with mbedtls, only optional, however as mbedtls
does not provide a pkg-config file, we assume that if zlib is
available, we must link with it to avoid a build failure when linking
statically with a zlib-enabled mbedtls.

This change was pushed upstream with
https://github.com/zhaojh329/rtty/commit/7b8efe11dbafce97971dc130bf6cc1756f34ce07
and is in buildroot since the bump to version 7.1.4 with commit
0c80245d.
However, this change will raise a build failure if ZLIB_LIBRARIES is
used when zlib is not found. This patch is fixing this build failure.

However, it should be noted that the compression support in mbedtls is
only enabled if BR2_PACKAGE_MBEDTLS_COMPRESSION=y. So we can have a
situation where mbedtls is enabled, zlib is enabled, but mbedtls is not
using zlib and as a result, since version 7.1.4, rttyt will needlessly
link with zlib in such a situation.

The only sane way to fix this is to use pkg-config, but as mbedtls
apparently doesn't provide any .pc file, we leave it as it is.

Fixes:
 - http://autobuild.buildroot.org/results/a0ebffe58bbf14cab74b7d2111d4d88a9c725273



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>