glsl: fix unreachable(!"") to unreachable("")

Correct error with commit 151fb1e where assert was renamed
to unreachable without removing ! from string argument.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/glsl/loop_controls.cpp

@@ -139,7 +139,7 @@ calculate_iterations(ir_rvalue *from, ir_rvalue *to, ir_rvalue *increment,
          iter = new(mem_ctx) ir_constant(double(iter_value + bias[i]));
          break;
       default:
-          unreachable(!"Unsupported type for loop iterator.");
+          unreachable("Unsupported type for loop iterator.");
       }
 
       ir_expression *const mul =