synergy: add dependency on openssl

* otherwise fails with:
  /home/jenkins/oe/world/shr-core/tmp-glibc/work/armv5te-oe-linux-gnueabi/synergy/1.7.3+1.7.4-rc8+AUTOINC+588fb4b805-r0/git/src/lib/plugin/ns/SecureSocket.cpp:26:25:
  fatal error: openssl/ssl.h: No such file or directory
  compilation terminated.
  make[2]: *** [src/lib/plugin/ns/CMakeFiles/ns.dir/SecureSocket.cpp.o]
  Error 1

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>