gitignore: ignore files generated by Automake's testsuite

These files are generated by Automake 1.13 when running the testsuite.
diff --git a/.gitignore b/.gitignore
index 8d7c74a..5bfab9a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,6 +16,7 @@
 /configure
 /libtool
 /stamp-h1
+/test-suite.log
 
 *~
 .*.swp