compiler: Add color system value

This is nice to have with radeonsi, where color varyings are handled
specially to avoid recompiles.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/compiler/shader_enums.c

@@ -231,6 +231,8 @@ gl_system_value_name(gl_system_value sysval)
      ENUM(SYSTEM_VALUE_SAMPLE_POS),
      ENUM(SYSTEM_VALUE_SAMPLE_MASK_IN),
      ENUM(SYSTEM_VALUE_HELPER_INVOCATION),
+     ENUM(SYSTEM_VALUE_COLOR0),
+     ENUM(SYSTEM_VALUE_COLOR1),
      ENUM(SYSTEM_VALUE_TESS_COORD),
      ENUM(SYSTEM_VALUE_VERTICES_IN),
      ENUM(SYSTEM_VALUE_PRIMITIVE_ID),