dev-perl/Crypt-Curve25519: Fix fmul conflicting types build breakage.

dev-perl/Crypt-Curve25519: Fix fmul conflicting types build breakage.

With thanks to Mike Benson for trawling the upstream bug reports:
https://github.com/ajgb/crypt-curve25519/issues/9#issuecomment-447845725

No revision bump as the package does not install without the fix.
This uses a patch rather than a sed statement, as the latter could fail
silently.

Closes: https://bugs.gentoo.org/669436
Suggested-By: Mike Benson <mike@kambe.com.au>
Signed-Off-By: Tony Vroon <chainsaw@gentoo.org>
Package-Manager: Portage-2.3.66, Repoman-2.3.11