The header was added by the recent keybuf feature (r316343)
MODINFOMD_KEYBUF originally resided here, but was moved to linker.h
This change fixes the build on risc-5 which doesn't have a metadata.h
Detected by Jenkins: https://ci.freebsd.org/job/FreeBSD-head-riscv64-build/1167/console
Reported by: lwhsu