btcutil: rename interfaceAddrs to InterfaceAddrs in appengine variant

The previous commit exported the interfaceAddrs helper in net.go but
missed the parallel definition in net_noop.go (build tag: appengine),
which left builds with -tags appengine broken with:

    ./certgen.go:79:16: undefined: InterfaceAddrs

Rename the appengine no-op variant to match, restoring the appengine
build.

btcutil/net_noop.go

@@ -11,10 +11,10 @@ import (
 	"net"
 )
 
-// interfaceAddrs returns a list of the system's network interface addresses.
+// InterfaceAddrs returns a list of the system's network interface addresses.
 // It is wrapped here so that we can substitute it for a no-op function that
 // returns an empty slice of net.Addr when building for systems that do not
 // allow access to net.InterfaceAddrs().
-func interfaceAddrs() ([]net.Addr, error) {
+func InterfaceAddrs() ([]net.Addr, error) {
 	return []net.Addr{}, nil
 }