fix: libflux build error on >= rust-1.72.0

[stkw0]

Closes: #5438

Checklist

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This a trivial patch to fix an issue with rust-1.72.0 (see linked issue).

[crepererum]

@stkw0 sorry for the late reply, could you please fix the commit message to follow our semantic commit style: fix: libflux build .... (basically just put a colon behind fix).