let cgo find versionl.h

pkg/zerotier/node.go

@@ -13,7 +13,7 @@
 
 package zerotier
 
-// #cgo CFLAGS: -O3
+// #cgo CFLAGS: -O3 -I${SRCDIR}/../../build/core
 // #cgo darwin LDFLAGS: -Wl,-undefined -Wl,dynamic_lookup
 // #cgo !darwin LDFLAGS: -Wl,-unresolved-symbols=ignore-all
 // #include "../../serviceiocore/GoGlue.h"