sparc64: Fix build regression

This patch fixes build error which was introduced by commit

812cb83a (Implement HAVE_CONTEXT_TRACKING).

[*]https://lkml.org/lkml/2013/11/23/103Signed-off-by: Kirill Tkhai <tkhai@yandex.ru>
CC: David Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc/kernel/kgdb_64.c

@@ -6,6 +6,7 @@
 #include <linux/kgdb.h>
 #include <linux/kdebug.h>
 #include <linux/ftrace.h>
+#include <linux/context_tracking.h>
 
 #include <asm/cacheflush.h>
 #include <asm/kdebug.h>