Add configure.ac to .gitignore

If the phpize script is part of the PHP 7.0 or 7.1 installation there
will be generated configure.in file from the PHP source code as a
part of the old autotools files. If the phpize script is from
PHP 7.2 or above there will be a new configure.ac file generated.
This file is further used to generate the configure script.

Having these files gitignored is useful when developing the extension.

.gitignore

@@ -20,6 +20,7 @@ config.status
 config.sub
 configure
 configure.in
+configure.ac
 *~
 install-sh
 libtool