libcamera: fix no-longer existent <libcamera/buffer.h> header inclusion

The SPA plugin is including a <libcamera/buffer.h> header file, but this
got renamed to <libcamera/framebuffer.h> to match the defined class name:

../spa/plugins/libcamera/libcamera_wrapper.cpp:52:10: fatal error: libcamera/buffer.h: No such file or directory
   52 | #include <libcamera/buffer.h>
      |          ^~~~~~~~~~~~~~~~~~~~

Fixes #1435