arb_shader_draw_parameters: Fix attribute input type
The shader uses a signed vector integer type as input, so lets also specify signed integer vertex array pointer input data. This is also the data type the given array is actually declared with.
While the OpenGL spec doesn't seem to care about such discrepancies, when running the piglits on Zink we actually end up with a validation error VUID-vkCmdDrawMultiIndexedEXT-Input-08734, because the vertex data type doesn't match the type specified in the shader.