cmake: add protolib to zmq dependency libs

Needed for Ubuntu 19.04:
https://ubuntu.pkgs.org/19.04/ubuntu-universe-amd64/libnorm-dev_1.5.8+dfsg2-1_amd64.deb.html

CMakeLists.txt

@@ -982,6 +982,7 @@ find_path(ZMQ_INCLUDE_PATH zmq.hpp)
 find_library(ZMQ_LIB zmq)
 find_library(PGM_LIBRARY pgm)
 find_library(NORM_LIBRARY norm)
+find_library(PROTOLIB_LIBRARY protolib)
 find_library(SODIUM_LIBRARY sodium)
 
 if(NOT ZMQ_INCLUDE_PATH)
@@ -996,6 +997,9 @@ endif()
 if(NORM_LIBRARY)
   set(ZMQ_LIB "${ZMQ_LIB};${NORM_LIBRARY}")
 endif()
+if(PROTOLIB_LIBRARY)
+  set(ZMQ_LIB "${ZMQ_LIB};${PROTOLIB_LIBRARY}")
+endif()
 if(SODIUM_LIBRARY)
   set(ZMQ_LIB "${ZMQ_LIB};${SODIUM_LIBRARY}")
 endif()