Fix reference to undefined target in CMakeLists.txt

Corrects bug where cmake fails on MacOS when building static-only
library with an error that looks like:

CMake Error at librabbitmq/CMakeLists.txt:143 (set_target_properties):
  set_target_properties Can not find target to add properties to: rabbitmq

Fixed: #796

Signed-off-by: GitHub <noreply@github.com>

librabbitmq/CMakeLists.txt

@@ -140,7 +140,7 @@ if(BUILD_STATIC_LIBS)
   )
 
   if (APPLE)
-    set_target_properties(rabbitmq PROPERTIES
+    set_target_properties(rabbitmq-static PROPERTIES
       MACHO_CURRENT_VERSION ${RMQ_SOVERSION}.${RMQ_SOVERSION_AGE}.${RMQ_SOVERSION_REVISION}
       MACHO_COMPATIBILITY_VERSION ${RMQ_SOVERSION}
     )