glsl: Re-synchronize ir_variable_mode and the printer's string array.

Since the introduction of ir_var_system_value, system variables would be
printed as "temporary" and temporaries would result in out-of-bounds
array access, showing up as garbage in printed IR.

src/glsl/ir_print_visitor.cpp

@@ -97,7 +97,7 @@ void ir_print_visitor::visit(ir_variable *ir)
    const char *const cent = (ir->centroid) ? "centroid " : "";
    const char *const inv = (ir->invariant) ? "invariant " : "";
    const char *const mode[] = { "", "uniform ", "in ", "out ", "inout ",
-			        "temporary " };
+			        "sys ", "temporary " };
    const char *const interp[] = { "", "flat", "noperspective" };
 
    printf("(%s%s%s%s) ",