Restore JV_PRINT_COLOUR as an alias

JV_PRINT_COLOUR was part of the public libjq headers and was removed as
part of 2d05b54. While JV_PRINT_COLOR is definitely the preferred
spelling this side of the pond, we shouldn't just remove otherwise
exposed enum values.

src/jv.h

@@ -198,7 +198,7 @@ int jv_get_refcnt(jv);
 enum jv_print_flags {
   JV_PRINT_PRETTY   = 1,
   JV_PRINT_ASCII    = 2,
-  JV_PRINT_COLOR    = 4,
+  JV_PRINT_COLOR    = 4, JV_PRINT_COLOUR = 4,
   JV_PRINT_SORTED   = 8,
   JV_PRINT_INVALID  = 16,
   JV_PRINT_REFCOUNT = 32,