Fix the automake rule to satisfy header requirements of atscppapi

When I built my plugin outside of trafficserver source tree, I found build failure related to header requirements of atscppapi as below logs.

```
# I set /usr/local/trafficserver/ as prefix.
In file included from /usr/local/trafficserver/include/atscppapi/Transaction.h:30:
/usr/local/trafficserver/include/atscppapi/shared_ptr.h:28:10: fatal error: 'ink_autoconf.h' file not found
#include "ink_autoconf.h"
         ^
1 error generated.
```

The shared_ptr.h requires a variable defined on ink_autoconf.h but it doesn't exist in destination directory. so I'll fix by installing it.