On musl libc, zfs failed to compile due to the missing <fcntl.h>
include, which is required for `open()` per POSIX.
This commit add the missing <fcntl.h> include.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Hiếu Lê <leorize+oss@disroot.org>
Closes#10880