Bump ffi from 1.9.18 to 1.12.1

[dependabot]

Bumps ffi from 1.9.18 to 1.12.1.

Changelog

Sourced from ffi's changelog.

1.12.1 / 2020-01-14

Added:

• Add binary gem support for ruby-2.7 on Windows

1.12.0 / 2020-01-14

Added:

• FFI::VERSION is defined as part of require 'ffi' now.
  It is no longer necessary to require 'ffi/version' .

Changed:

• Update libffi to latest master.

Deprecated:

• Overwriting struct layouts is now warned and will be disallowed in ffi-2.0. #734, #735

1.11.3 / 2019-11-25

Removed:

• Remove support for tainted objects which cause deprecation warnings in ruby-2.7. #730

1.11.2 / 2019-11-11

Added:

• Add DragonFlyBSD as a platform. #724

Changed:

• Sort all types.conf files, so that files and changes are easier to compare.
• Regenerated type conf for freebsd12 and x86_64-linux targets. #722
• Remove MACOSX_DEPLOYMENT_TARGET that was targeting very old version 10.4. #647
• Fix library name mangling for non glibc Linux/UNIX. #727
• Fix compiler warnings raised by ruby-2.7
• Update libffi to latest master.

1.11.1 / 2019-05-20

Changed:

• Raise required ruby version to >=2.0. #699, #700
• Fix a possible linker error on ruby < 2.3 on Linux.

... (truncated)

Commits

• b54e735 Bump VERSION to 1.12.1
• ee13af1 Do parallel builds in CI docker containers
• 2c81c6b Respect MAKE environment variable for libtest build
• b2793a3 Better document enable/disable libffi switches in the README
• 8169aef Bump VERSION to 1.12.0
• 62c0c97 Update CHANGELOG for version 1.12.0
• da4402f Update libffi to latest master branch
• d564eb4 Merge pull request #735 from larskanis/disallow-re-layout
• 89f4258 Warn about Struct layout redefinition only
• bd45ff5 Namespace all benchmarks since they are loaded into one process
• Additional commits viewable in compare view

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