smpp: Fix the make distcheck for smpp

DISTCHECK_CONFIGURE_FLAGS="--enable-smpp" make distcheck
diff --git a/openbsc/src/libmsc/Makefile.am b/openbsc/src/libmsc/Makefile.am
index c36ba92..2b50599 100644
--- a/openbsc/src/libmsc/Makefile.am
+++ b/openbsc/src/libmsc/Makefile.am
@@ -19,5 +19,6 @@
 			osmo_msc.c
 
 if BUILD_SMPP
+noinst_HEADERS = smpp_smsc.h
 libmsc_a_SOURCES += smpp_smsc.c smpp_openbsc.c smpp_vty.c smpp_utils.c
 endif