std.net.curl: add missing import to examples

[pbackus]

Fixes issue 23724 - HTTP.onReceive example does not compile

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23724	minor	HTTP.onReceive example does not compile

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8688"

[CyberShadow]

LGTM.

I would also change the to call to UFCS, I think today that is more idiomatic.