Add apt-pkg/tagfile-keys.cc to the gitignore file

This is output of triehash.

Gbp-Dch: ignore

.gitignore

@@ -17,6 +17,7 @@ gtest-*
 *.o
 *.so*
 *.versionscript
+**/apt-pkg/tagfile-keys.cc
 
 # Templates
 *.pot