Define SER_MOD_INTERFACE in Makefile.

This patch adds -DSER_MOD_INTERFACE to the modules Makefile, this
is needed to make ser modules compile with the sip router core.

modules/tls/Makefile

@@ -17,4 +17,6 @@ LIBS+= -L$(LOCALBASE)/lib -L$(LOCALBASE)/ssl/lib -lssl  -lcrypto \
 #       E.g.: make TLS_HOOKS=1 TLS_EXTRA_LIBS="-lz -lkrb5"
 
 
+DEFS+=-DSER_MOD_INTERFACE
+
 include ../../Makefile.modules