include: add missing ipa.h to Makefile.am so make distcheck works again
diff --git a/include/osmocom/abis/Makefile.am b/include/osmocom/abis/Makefile.am
index df70de3..5d47363 100644
--- a/include/osmocom/abis/Makefile.am
+++ b/include/osmocom/abis/Makefile.am
@@ -2,6 +2,7 @@
 		   e1_input.h		\
 		   lapd.h		\
 		   subchan_demux.h	\
+		   ipa.h		\
 		   ipa_proxy.h		\
 		   ipaccess.h		\
 		   trau_frame.h