Makefile.am: fix pkgconfig_DATA

Fix building debian packages:
	dh_install: Cannot find (any matches for) "usr/lib/*/pkgconfig/libosmo-mslookup.pc" (tried in "." and "debian/tmp")
	dh_install: libosmo-mslookup-dev missing files: usr/lib/*/pkgconfig/libosmo-mslookup.pc
	dh_install: missing files, aborting

Fixes: bf7deda0fc30 ("add libosmo-mslookup abstract client")
Change-Id: Ib0bce2d09b41834f7331969eaf7c57a9787f7efb
diff --git a/Makefile.am b/Makefile.am
index a612bf4..a54dcb2 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -17,7 +17,8 @@
 	--with-systemdsystemunitdir=$$dc_install_base/$(systemdsystemunitdir)
 
 pkgconfigdir = $(libdir)/pkgconfig
-pkgconfig_DATA = libosmo-gsup-client.pc
+pkgconfig_DATA = libosmo-gsup-client.pc \
+		 libosmo-mslookup.pc
 
 @RELMAKE@