Fix Core build with SPARK_NO_CLOUD

Author: sergeuz

system/src/system_cloud_internal.cpp

@@ -1121,6 +1121,8 @@ CloudDiagnostics* CloudDiagnostics::instance() {
 
 namespace particle {
 
+#ifndef SPARK_NO_CLOUD
+
 bool publishKeepaliveInterval(unsigned interval) {
     if (!interval) {
         // Get the current interval
@@ -1138,4 +1140,6 @@ bool publishKeepaliveInterval(unsigned interval) {
     return publishEvent(KEEPALIVE_INTERVAL_EVENT, buf);
 }
 
+#endif // !defined(SPARK_NO_CLOUD)
+
 } // particle