Add return statement in main

Add return statement in main() to supress issue reported by static
code analysis tool.

Signed-off-by: Arun P. Mohanan <arun.p.m@linux.intel.com>
Change-Id: I83647b8e44b3ffa1912abc6815346988bc379830
diff --git a/src/mainapp.cpp b/src/mainapp.cpp
index 14d352f..bde3f23 100644
--- a/src/mainapp.cpp
+++ b/src/mainapp.cpp
@@ -27,4 +27,6 @@
     UBootEnvMgr uBootEnvMgr(io, server, conn);
 
     io.run();
+
+    return 0;
 }