Musl libc's <stdio.h> doesn't include <stdarg.h>, which cause
`va_start` and `va_end` end up being undefined symbols.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Leorize <alaviss@users.noreply.github.com>
Closes#6310
bf04e4d442