build: distcheck: add missing distclean files

Change-Id: I24d72b2b1bae52a1b2cf8a989396d2aac31d119e
diff --git a/include/osmocom/rua/Makefile.am b/include/osmocom/rua/Makefile.am
index 8e8896d..49e2150 100644
--- a/include/osmocom/rua/Makefile.am
+++ b/include/osmocom/rua/Makefile.am
@@ -35,3 +35,6 @@
 	RUA_TriggeringMessage.h \
 	RUA_TypeOfError.h \
 	RUA_UnsuccessfulOutcome.h
+
+DISTCLEANFILES = \
+	rua_ies_defs.h