uhttpd: do not configure TLS parameters if libustream-ssl is not present

A quite frequent problem after sysupgrading from an older, SSL enabled build
is that ustream-ssl is not installed so uhttpd fails to come up again due to
https listening directives in the preserved configuration.

Skip key/cert and ssl listen options when libustream-ssl.so is not present.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42284 3c298f89-4303-0410-b956-a3cf2f4a3e73