net: add -lsendfile to cgo LDFLAGS for solaris

Fixes external linking of net/http tests (or anything that uses
sendfile).

Fixes #12390.

Change-Id: Iee08998cf66e7b0ce851db138a00ebae6dc2395e
Reviewed-on: https://go-review.googlesource.com/14072Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>

src/net/cgo_solaris.go

@@ -7,7 +7,7 @@
 package net
 
 /*
-#cgo LDFLAGS: -lsocket -lnsl
+#cgo LDFLAGS: -lsocket -lnsl -lsendfile
 #include <netdb.h>
 */
 import "C"