x11-clocks/glclock: Fix build with Clang 6

GLObject.cpp:843:22: error: non-constant-expression cannot be narrowed from type 'double' to 'GLfloat' (aka 'float') in initializer list [-Wc++11-narrowing]

GLfloat lamb[4] = { color[0] * 0.1, color[1] * 0.1, color[2] * 0.1, 1.0f } ;
                    ^~~~~~~~~~~~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p474051_s336054/logs/glclock-6.0.b6.0_6.log