util/anon_file: const string param

Fixes: c0376a12 ("util: add anon_file.h for all memfd/temp file usage")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Tested-by: Eric Anholt <eric@anholt.net>
Tested-by: Andreas Baierl <ichgeh@imkreisrum.de>
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