glsl: improve the accuracy of the radians() builtin function

glsl: improve the accuracy of the radians() builtin function

The constant used in the radians() function didn't have enough
precision, causing a relative error of 1.676e-5, which is far worse
than the precision of 32-bit floats.  This patch reduces the relative
error to 1.14e-9, which is the best we can do in 32 bits.

Fixes piglit tests {fs,vs}-radians-{float,vec2,vec3,vec4}.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>