Make dependency on properties-cpp explicit

...so its include files are found even in a non-default location

src/CMakeLists.txt

@@ -8,6 +8,7 @@ include_directories(
   ${DBUS_INCLUDE_DIRS}
   ${SDL2_INCLUDE_DIRS}
   ${SDL2_IMAGE_INCLUDE_DIRS}
+  ${PROPERTIES_CPP_INCLUDE_DIRS}
   ${LXC_INCLUDE_DIRS}
   ${MIRCLIENT_INCLUDE_DIRS}
   ${CMAKE_CURRENT_BINARY_DIR}