MFH: r481271

devel/rust-cbindgen: update to 0.6.4

Changes: https://github.com/eqrion/cbindgen/releases/tag/v0.6.4
Changes: https://github.com/eqrion/cbindgen/compare/v0.6.3...v0.6.4
Reported by: portscout
Approved by: ports-secteam blanket (required for Firefox 63)