libshare: Add missing headers for nfs.c

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

lib/libshare/os/linux/nfs.c

@@ -31,6 +31,7 @@
 #include <string.h>
 #include <strings.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <sys/file.h>
 #include <sys/stat.h>
 #include <sys/types.h>