Correct option number for DHCP_DNS_SERVERS, and add it to the request

list.

src/include/gpxe/dhcp.h

 #define DHCP_ROUTERS 3
 
 /** DNS servers */
-#define DHCP_DNS_SERVERS 4
+#define DHCP_DNS_SERVERS 6
 
 /** Host name */
 #define DHCP_HOST_NAME 12

src/net/udp/dhcp.c

 	DHCP_STRING (  'E', 't', 'h', 'e', 'r', 'b', 'o', 'o', 't' ),
 	DHCP_PARAMETER_REQUEST_LIST,
 	DHCP_OPTION ( DHCP_SUBNET_MASK, DHCP_ROUTERS, DHCP_HOST_NAME,
-		      DHCP_BOOTFILE_NAME, DHCP_EB_ENCAP ),
+		      DHCP_BOOTFILE_NAME, DHCP_DNS_SERVERS, DHCP_EB_ENCAP ),
 	DHCP_END
 };