BUG: function pointers inside a struct do not generate SegmentAllocators when needed

[Marko]

Hello again!
When trying to call a function pointer inside a struct which returns a
MemorySegment, I receive the following error:
```
Exception in thread "main" java.lang.AssertionError: should not reach here
at Foo$bar.lambda$ofAddress$0(Foo.java:34)
at Main.main(Main.java:8)
Caused by: java.lang.invoke.WrongMethodTypeException: expected
(NativeSymbol,SegmentAllocator)MemorySegment but found
(NativeSymbol)MemorySegment
at java.base/java.lang.invoke.Invokers.newWrongMethodTypeException(Invokers.java:523)
at java.base/java.lang.invoke.Invokers.checkExactType(Invokers.java:532)
at Foo$bar.lambda$ofAddress$0(Foo.java:32)
... 1 more
```
I believe this is due to
`FunctionalInterfaceBuilder#emitFunctionalFactoryForPointer` not
checking to see if the returnType is a MemorySegment, as it does in
`HeaderFileBuilder` for regular functions, see
https://github.com/openjdk/jextract/blob/0582eaf1b4cdba95f0ee8c2480767433bb647d0d/src/main/java/org/openjdk/jextract/impl/HeaderFileBuilder.java#L122-L127.

I began implementing a fix by copying this functionality over, however
there is a bit of an issue I came across. When jextract emits the
getter for the functional interface in a struct (StructBuilder), only
the name of the interface is known. AFAIK, it does not know whether it
needs a SegmentAllocator and cannot currently generate the correct
code.

I thought about adding another field to the method
`emitFunctionalFactoryForPointer`, similar to fiName, like a boolean
for whether a segment allocator is needed or not, however, I can't help but
feel it is a bit hacky. I wonder what your opinions are on this.

A repro can be found here:
https://github.com/lost-illusi0n/jextract-func-ptr-seg-alloc-repro
Just run the run script if you are on Linux. You will have to change the
library output in the script if you are on another os.