dev-libs/libffi: drop libffi version from includedir, bug #695788

dev-libs/libffi: drop libffi version from includedir, bug #695788

Posting ebuild source note as-is:

```
 --includedir= path maintains a few properties:
 1. have stable name across libffi versions: some packages like
    dev-lang/ghc or kde-frameworks/networkmanager-qt embed
    ${includedir} at build-time. Don't require those to be
    rebuilt unless SONAME changes. bug #695788

    We use /usr/.../${PN} (instead of former /usr/.../${P}).

 2. have ${ABI}-specific location as ffi.h is target-dependent.

    We use /usr/$(get_libdir)/... to have ABI identifier.
```

Before the change libffi installed headers into:
    /usr/lib/libffi-3.3/include/
    /usr/lib64/libffi-3.3/include/
After the change the patchs are:
    /usr/lib/libffi/include/
    /usr/lib64/libffi/include/

Reported-by: Franz Trischberger
Bug: https://bugs.gentoo.org/695788
Package-Manager: Portage-2.3.79, Repoman-2.3.18
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>