Update .gitignore so that the newly-added config.h is visible to git.

Change-Id: I0054c8d6f80fc71073ca70a121f1acafc12e030b
diff --git a/.gitignore b/.gitignore
index f0f1f29..2ccf83f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -24,7 +24,7 @@
 # in-tree configure-generated files
 Makefile
 src/Makefile
-config.h
+/config.h
 config.log
 config.status