gallium/winsys/nouveau: Don't mark the api PUBLIC

We don't want to export this symbol from our shared object - any use of
this function from outside of mesa (like the ddx) should get the version
from libdrm, not the private copy in mesa.

Fixes: 821f4c8d99 ("nouveau: import libdrm_nouveau")
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32740>

src/gallium/winsys/nouveau/drm/nouveau_drm_winsys.c

@@ -82,7 +82,7 @@ err_dev_new:
    return NULL;
 }
 
-PUBLIC struct pipe_screen *
+struct pipe_screen *
 nouveau_drm_screen_create(int fd)
 {
    return u_pipe_screen_lookup_or_create(os_dupfd_cloexec(fd), NULL, NULL,