Disable CLI on windows

UNIT_ign_TEST was disabled on windows anyway. In addition, this follows
the pattern in ign-transport and ign-msgs.

Signed-off-by: Addisu Z. Taddese <addisu@openrobotics.org>

src/CMakeLists.txt

@@ -77,5 +77,7 @@ if(TARGET UNIT_ign_TEST)
     ENVIRONMENT "${_env_vars}")
 endif()
 
-add_subdirectory(cmd)
+if(NOT WIN32)
+  add_subdirectory(cmd)
+endif()
 add_subdirectory(plugins)