wgl: Fix unintentional assignment on assert.

Spotted by Nanley Chery.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4427
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Neha Bhende <bhenden@vmware.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22706>

src/gallium/frontends/wgl/stw_tls.c

@@ -306,7 +306,7 @@ stw_tls_get_data(void)
    }
 
    assert(data);
-   assert(data->dwThreadId = GetCurrentThreadId());
+   assert(data->dwThreadId == GetCurrentThreadId());
    assert(data->next == NULL);
 
    return data;