reverting unintended change in r14897 (3rdparty line removed)

git-svn-id: https://svn.code.sf.net/p/rathena/svn/trunk@14898 54d463be-8e91-2dee-dedb-b68131a5f0ec

CMakeLists.txt

@@ -285,4 +285,5 @@ endif()
 #
 # subdirectories
 #
+add_subdirectory( 3rdparty )
 add_subdirectory( src )