Makefile.am: Make libraries depend on .map files

Without this, an update to the .map file doesn't re-trigger a re-link of
the library.

Change-Id: If0ea6f41730a28b04562d45efa6ca376abaf3f6b
diff --git a/src/coding/Makefile.am b/src/coding/Makefile.am
index 42b16e8..905d684 100644
--- a/src/coding/Makefile.am
+++ b/src/coding/Makefile.am
@@ -37,3 +37,4 @@
 	$(NULL)
 
 EXTRA_DIST = libosmocoding.map
+EXTRA_libosmocoding_la_DEPENDENCIES = libosmocoding.map