Add back <err.h> for warn().

(of course it's pretty stupid to call it, as printf doesn't work w/in
sysinstall, and sysinstall is the only consumer of libdisk)

lib/libdisk/disk.c

@@ -15,6 +15,7 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <string.h>
+#include <err.h>
 #include <sys/sysctl.h>
 #include <sys/types.h>
 #include <sys/stat.h>