Doxygen: enable tagfile generation and usage for cross-project references

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/Doxyfile.core.in b/Doxyfile.core.in
index 045402f..58d35be 100644
--- a/Doxyfile.core.in
+++ b/Doxyfile.core.in
@@ -1490,7 +1490,7 @@
 # When a file name is specified after GENERATE_TAGFILE, doxygen will create
 # a tag file that is based on the input files it reads.
 
-GENERATE_TAGFILE       =
+GENERATE_TAGFILE       = doc/libosmocore.tag
 
 # If the ALLEXTERNALS tag is set to YES all external classes will be listed
 # in the class index. If set to NO only the inherited external classes