cosmetic: configure.ac: Fix tabulation in line

Change-Id: I5cfc90ace5f9cc9c3fe4dde7aeccbdf1909da007
diff --git a/configure.ac b/configure.ac
index 479ae3b..2cf44fc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -258,7 +258,7 @@
                  intl/Makefile
                  po/Makefile
                  sgsnemu/Makefile
-    doc/manuals/Makefile
+                 doc/manuals/Makefile
                  contrib/Makefile
                  contrib/systemd/Makefile
                  contrib/osmo-ggsn.spec