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/gb/Makefile.am b/src/gb/Makefile.am
index 1818db7..3a64a71 100644
--- a/src/gb/Makefile.am
+++ b/src/gb/Makefile.am
@@ -40,3 +40,4 @@
 endif
 
 EXTRA_DIST = libosmogb.map
+EXTRA_libosmogb_la_DEPENDENCIES = libosmogb.map