wined3d: Make the device parameter to device_unit_free_for_vs() const.

dlls/wined3d/device.c

diff --git a/dlls/wined3d/device.c b/dlls/wined3d/device.c
index 1dc6cbc..43d51e7 100644
--- a/dlls/wined3d/device.c
+++ b/dlls/wined3d/device.c
@@ -2954,7 +2954,7 @@
     }
 }
 
-static BOOL device_unit_free_for_vs(struct wined3d_device *device,
+static BOOL device_unit_free_for_vs(const struct wined3d_device *device,
         const WINED3DSAMPLER_TEXTURE_TYPE *pshader_sampler_tokens,
         const WINED3DSAMPLER_TEXTURE_TYPE *vshader_sampler_tokens, DWORD unit)
 {