pkg/flatrpc: run make generate #18399

Job	Run time
aux	4m 25s
build	4m 7s
race	7m 6s
gvisor	4m 3s
dashboard	2m 13s
old	7m 22s
arch (presubmit_arch_windows)	3m 12s
arch (presubmit_arch_darwin)	3m 43s
arch (presubmit_arch_executor)	3m 30s
arch (presubmit_arch_linux)	10m 11s
arch (presubmit_arch_freebsd)	5m 57s
race_dashboard	3m 17s
arch (presubmit_arch_openbsd)	3m 42s
arch (presubmit_arch_netbsd)	3m 56s
	1h 6m 44s

Provide feedback