fstab-generator: quiesce false-positive -Werror=format-nonliteral (#5336)

fstab-generator: quiesce false-positive -Werror=format-nonliteral (#5336)

Commit ae3251851 changed the fprintf() format argument into a variable
which triggers a gcc 6.3 warning/error:

  src/fstab-generator/fstab-generator.c:243:17: error: format not a string literal,
  argument types not checked [-Werror=format-nonliteral]
                 fprintf(f, format, res);

This is a false positive, as the function is only being called with
constant (not user-definable) arguments which are valid format strings.