cmake: mark readline include directories as SYSTEM (thanks, Tobias!).

CMakeLists.txt

@@ -204,7 +204,7 @@ if(PHYSFS_BUILD_TEST)
         if(READLINE_LIBRARY)
             set(HAVE_SYSTEM_READLINE TRUE)
             set(TEST_PHYSFS_LIBS ${TEST_PHYSFS_LIBS} ${READLINE_LIBRARY} ${CURSES_LIBRARY})
-            include_directories(${READLINE_H} ${HISTORY_H})
+            include_directories(SYSTEM ${READLINE_H} ${HISTORY_H})
             add_definitions(-DPHYSFS_HAVE_READLINE=1)
         endif()
     endif()