build(deps): point third-party/build-deps at prores-videotoolbox encoder enablement

Carries the FFmpeg configure flag change (LizardByte/build-deps#693,
commit c40fba6 "build: enable ProRes VideoToolbox encoder") that the
ProRes runtime wiring in this PR depends on.

The SHA a51ecf15 lives on the RESMP-DEV/build-deps fork branch
feat/ffmpeg/enable-prores-videotoolbox-encoder — it is not yet on
LizardByte/build-deps master because build-deps#693 is still open.

Normally this would ship as a standalone `build(deps): bump
third-party/build-deps` PR per the Sunshine convention (cf. #5296,
#5280, #5271), but those PRs always point at canonical master SHAs.
Until build-deps#693 lands, this fork-scoped pointer is folded into
the feature PR so reviewers can actually build and test the ProRes
path. Once #693 merges, replace a51ecf15 with the canonical master
SHA via the standard dep-bump flow.

Author: Nottlespike