synergy: add dependency on curl

* otherwise fails with:
  | -- Looking for pthread_create in pthread
  | -- Looking for pthread_create in pthread - found
  | -- Could NOT find CURL (missing:  CURL_LIBRARY CURL_INCLUDE_DIR)
  | CMake Error at CMakeLists.txt:165 (message):
  |   Missing library: curl
  |
  |
  | -- Configuring incomplete, errors occurred!

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