Makefile.am: add 'src/s390/internal.h' to source tarball

commit 2f530de168e0253ac06e044c832132c496e8788b
("s390: Reorganize assembly") introduced new header
(similar to other arches) but did not add it to source
tarball.

As a result build from 'make dist' tarballs failed as:

```
../src/s390/ffi.c:34:10: fatal error: internal.h: No such file or directory
 #include "internal.h"
          ^~~~~~~~~~~~
```

To fix it the change adds file to 'Makefile.am'.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
1 file changed