Fix compile error when CONFIG_BOOTP_RANDOM_DELAY is set.

The option CONFIG_BOOTP_RANDOM_DELAY does not compile, because of a
missing extern inside the net/bootp.h header

Signed-off-by: Remy Bohmer <linux@bohmer.net>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>

net/bootp.h

@@ -60,7 +60,7 @@ extern ulong	BootpID;		/* ID of cur BOOTP request		*/
 extern char	BootFile[128];		/* Boot file name			*/
 extern int	BootpTry;
 #ifdef CONFIG_BOOTP_RANDOM_DELAY
-ulong		seed1, seed2;		/* seed for random BOOTP delay		*/
+extern ulong	seed1, seed2;		/* seed for random BOOTP delay		*/
 #endif