Add dummy symbol for static library generation

libapps is used internally for the linkage of user applications, and
there should be at least one symbol even when all applications are
absent via top-level configuration. Otherwise, the ranlib would complain
as following:
  ranlib: file: .libapps.a/libapps.a(dummy.c.o) has no symbols

Author: jserv

apps/dummy.c

@@ -1 +1,2 @@
 /* Placeholder */
+int _apps_dummy;