discover/device-handler: Include makedev() from sysmacros.h

discover/device-handler: Include makedev() from sysmacros.h

Include sys/sysmacros.h explicitly in response to the following error
message:

../discover/device-handler.c:1001:13: warning: In the GNU C Library, "makedev" is defined
 by <sys/sysmacros.h>. For historical compatibility, it is
 currently defined by <sys/types.h> as well, but we plan to
 remove this soon. To use "makedev", include <sys/sysmacros.h>
 directly. If you did not intend to use a system-defined macro
 "makedev", you should undefine it after including <sys/types.h>.
  id = makedev(1, handler->n_ramdisks);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>