Add support for vips 8.16.0

[der-eismann]

When trying to compile our application with vips 8.16.0 & govips 2.15.0 I get the following error:

# github.com/davidbyttow/govips/v2/vips
vendor/github.com/davidbyttow/govips/v2/vips/foreign.go:23:43: could not determine kind of name for C.VIPS_FOREIGN_JPEG_SUBSAMPLE_AUTO
vendor/github.com/davidbyttow/govips/v2/vips/foreign.go:26:43: could not determine kind of name for C.VIPS_FOREIGN_JPEG_SUBSAMPLE_LAST
vendor/github.com/davidbyttow/govips/v2/vips/foreign.go:25:43: could not determine kind of name for C.VIPS_FOREIGN_JPEG_SUBSAMPLE_OFF
vendor/github.com/davidbyttow/govips/v2/vips/foreign.go:24:43: could not determine kind of name for C.VIPS_FOREIGN_JPEG_SUBSAMPLE_ON
vendor/github.com/davidbyttow/govips/v2/vips/foreign.go:355:20: could not determine kind of name for C.VipsForeignJpegSubsample
cgo: 
clang errors for preamble:
In file included from vendor/github.com/davidbyttow/govips/v2/vips/foreign.go:3:
./foreign.h:87:3: error: unknown type name 'VipsForeignJpegSubsample'; did you mean 'VipsForeignSubsample'?
   87 |   VipsForeignJpegSubsample jpegSubsample;
      |   ^~~~~~~~~~~~~~~~~~~~~~~~
      |   VipsForeignSubsample
/thumbs/include/vips/foreign.h:475:3: note: 'VipsForeignSubsample' declared here
  475 | } VipsForeignSubsample;
      |   ^
1 error generated.

Looks like the enums were removed in this commit: libvips/libvips@a935ba7

Would be great if you could make your lib work with the newest vips version 🙂

[tonimelisma]

Thanks for catching this @der-eismann! Do you think you would have the time to contribute a fix for this?

I would love to accept a PR if one is forthcoming.

Thank you!

[cdimitroulas]

@tonimelisma I opened a PR for this, but given my lack of experience writing C, header files and CGo I have no idea if this is the right approach 😄 Would appreciate if you could take a look and let me know if this is what you expected.