audio/fluidsynth: Update to 2.5.6

Fixes multiple security issues:

Fix CVE-2026-58264 - a heap-based buffer overrun in command handler
Fix a heap-based buffer overflow in MIDI player
Fix a heap-based buffer overrun for DLS samples
Fix a DLS ptbl chunk integer overflow
Fix a DLS articulation chunk integer overflow
Fix a SF2 DMOD chunk integer underflow

Changelog: https://github.com/FluidSynth/fluidsynth/releases/tag/v2.5.6

Security: CVE-2026-58264