trace_skb: fix build when CONFIG_NET is not enabled

Fix trace_skb_sources build when CONFIG_NET is not enabled:

kernel/built-in.o: In function `probe_skb_dequeue':
trace_skb_sources.c:(.text+0xd9152): undefined reference to `init_net'
trace_skb_sources.c:(.text+0xd9188): undefined reference to `dev_get_by_index'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/trace/Kconfig

@@ -236,6 +236,7 @@ config BOOT_TRACER
 
 config SKB_SOURCES_TRACER
 	bool "Trace skb source information"
+	depends on NET
 	select GENERIC_TRACER
 	help
 	   This tracer helps developers/sysadmins correlate skb allocation and