Include errno.h in the few drivers which use it, rather than in nic.h

src/include/nic.h

@@ -11,7 +11,6 @@
 #include <stdint.h>
 #include <string.h>
 #include <stdio.h>
-#include <errno.h>
 #include <byteswap.h>
 #include <gpxe/pci.h>
 #include <gpxe/isapnp.h>