games/kuklomenos: Fix build with Clang 6

background.cc:69:6: error: non-constant-expression cannot be narrowed from type 'unsigned int' to 'int' in initializer list [-Wc++11-narrowing]

(colour & 0xff000000) >> 24,
^~~~~~~~~~~~~~~~~~~~~~~~~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p475478_s336801/logs/errors/kuklomenos-0.4.5_4.log