Compile error (on MacOS)

[Frank de Jong]

I have been able to successfully build ART for a while but with the latest source code I ran into a compilation error:

/Users/frank/src/art/rtengine/ipcolorcorrection.cc:331:46: error: non-const reference cannot bind to vector element

Color::hsl2yuv(h, s, u[k], v[k]);

^~~~

/Users/frank/src/art/rtengine/color.h:821:50: note: passing argument to parameter 'u' here

static void hsl2yuv(float h, float s, float &u, float &v);

^

[agriggio]

color correction: fixed compilation error on OS X

Fixes #215

→ <<cset 09ab1960b3bb>>