buildroot

libfcgi: Use SPDX license identifier

The fcgi license is covered by SPDX, the identifier however is not
obvious.

For details, see https://spdx.org/licenses/OML.html

[Peter: add spdx.org link]
Signed-off-by: Alexander Dahl <post@lespocky.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>