`ffi.enable` check incorrect, default value passes it, even if not enabled

[ping-localhost]

Was trying to resolve why I kept getting this error, even though I was certain that I installed the extension properly:

Unable to open library 'libvips.42.dylib' in any of ['/opt/homebrew/lib/']. Make sure that you've installed libvips and that 'libvips.42.dylib' is on your system's library search path.

So I added a debugger and the actual error being thrown was: FFI API is restricted by "ffi.enable" configuration directive.

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Since ini_get('ffi.enable') returns preload instead of false it passes the following check:

php-vips/src/FFI.php

Line 284 in 6dbba1b

if (!ini_get('ffi.enable')) {

This is incorrect and should be changed to check for an actual true.

[jcupitt]

Thanks for the report @ping-localhost

I made a quick PR and credited you. Does it look OK?