cairo.h is not found in ZynAddSubFX

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### System Information

Arch Linux (x86_64), Niri 26.04 - 11th Gen Intel(R) Core(TM) i3-1115G4, 12 GB RAM

### LMMS Version(s)

4e677cb6c

### Most Recent Working Version

_No response_

### Bug Summary

Does not compile because of missing headers.

Yes, I have cairo. `$ ls /usr/include/cairo/` returns
```
cairo-deprecated.h
cairo-features.h
cairo-ft.h
cairo-gobject.h
cairo.h
...
```

*[Assumption]
Might be possible to fix by just adding the right flags (I don't see cairo on CMakeLists.txt for ZynAddSubFX?????????)

### Expected Behaviour

To compile with no issues

### Steps To Reproduce

git clone https://github.com/lmms/lmms.git
cd lmms
cmake -S . -B build
cd build
make

### Logs

*COMPILE LOGS

Click to expand

In file included from /usr/include/FL/Fl.H:30,

                 from /home/aidan/lmms/build/plugins/ZynAddSubFx/zynaddsubfx/src/UI/ADnoteUI.h:5,

                 from /home/aidan/lmms/build/plugins/ZynAddSubFx/zynaddsubfx/src/UI/ADnoteUI.cxx:3:

/usr/include/FL/Fl_Cairo.H:40:11: fatal error: cairo.h: No such file or directory

   40 | # include <cairo.h>

      |           ^~~~~~~~~

### Screenshots / Minimum Reproducible Project

### Please search the issue tracker for existing bug reports before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start with the ZynAddSubFX build configuration and the reported include path plugins/ZynAddSubFx/zynaddsubfx/src/UI/ADnoteUI.cxx, where Fl_Cairo.H includes cairo.h. Reproduce the failure with the listed cmake and make commands, then verify that the project builds successfully with the Cairo headers found under /usr/include/cairo/.

Written by the indexing model from the issue text.