vc4: Fix the shader record size for extended strides.

It turns out they aren't packed when attributes are missing, according to
both docs and simulation.

src/gallium/drivers/vc4/vc4_simulator_validate.c

@@ -90,6 +90,7 @@ gl_shader_rec_size(uint32_t pointer_bits)
 {
 	uint32_t attribute_count = pointer_bits & 7;
 	bool extended = pointer_bits & 8;
+	uint32_t size;
 
 	if (attribute_count == 0)
 		attribute_count = 8;