Move common parts of libext{4,6}.a into libext.a

Signed-off-by: Maciej Zenczykowski <maze@google.com>
diff --git a/.gitignore b/.gitignore
index e5d3099..9b59e6a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -13,6 +13,7 @@
 Makefile.in
 
 /extensions/GNUmakefile
+/extensions/initext.c
 /extensions/initext?.c
 /extensions/matches?.man
 /extensions/targets?.man