check_dhcp.c: Fix constant convertion warning

check_dhcp.c:520:34: warning: implicit conversion from 'int' to 'char' changes
      value from 255 to -1 [-Wconstant-conversion]
        discover_packet.options[opts++]=DHCP_OPTION_END;
                                       ~^~~~~~~~~~~~~~~
check_dhcp.c:189:41: note: expanded from macro 'DHCP_OPTION_END'
                                        ^~~
1 warning generated.

plugins-root/check_dhcp.c

@@ -516,8 +516,8 @@ int send_dhcp_discover(int sock){
 		discover_packet.options[opts++]='\x04';
 		memcpy(&discover_packet.options[opts],&requested_address,sizeof(requested_address));
 		opts += sizeof(requested_address);
-	        }
-	discover_packet.options[opts++]=DHCP_OPTION_END;
+		}
+	discover_packet.options[opts++] = (char) DHCP_OPTION_END;
 
 	/* unicast fields */
 	if(unicast)