dev-libs/ffcall: use einfo, not elog in src_prepare()

dev-libs/ffcall: use einfo, not elog in src_prepare()

Noticed the post-merge output from dev-libs/ffcall when
was installing gforth. The output did not look useful:

```
 * Messages for package dev-libs/ffcall-2.1:

 * Patching 'Makefile.in'
 * Patching 'avcall/Makefile.in'
 * Patching 'callback/Makefile.in'
 * Patching 'gnulib-lib/Makefile.in'
 * Patching 'trampoline/Makefile.in'
 * Patching 'vacall/Makefile.in'
 * Patching 'callback/trampoline_r/Makefile.in'
 * Patching 'callback/vacall_r/Makefile.in'
 * Patching 'Makefile.in'
```

Package-Manager: Portage-2.3.94, Repoman-2.3.21
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>