Fix pg_walsummary's .gitignore

It missed a entry for tmp_check/ generated by the tests.  While on it,
append a slash at the beginning of "pg_walsummary" to restrict its check
to the current directory, like anywhere else.

Oversights in ee1bfd1683.

src/bin/pg_walsummary/.gitignore

@@ -1 +1,2 @@
-pg_walsummary
+/pg_walsummary
+/tmp_check/