fluidsynth: update to 2.5.4

fluidsynth: update to 2.5.4

Update spans 2.4.7 -> 2.5.4. Highlights:
 * 2.5.x: Major version with API additions for handling sequencer
   client unregistration, MIDI file reading flexibility, soundfont
   selectors and sample tuning improvements.
 * Various bug fixes for SF3 voice handling, reverb engine
   stability, GM/GS/XT mode reset behaviour and audio drivers
   (PortAudio, SDL2, Pulseaudio, JACK, OPL).
 * Build system fixes including CMake updates and new toolchain
   compatibility.

fluidsynth >= 2.5 requires GCEM (a header-only constexpr math
library) at build time. Build-depend on the new 'gcem' package,
which installs the headers and CMake config files into staging
where find_package(GCEM REQUIRED) picks them up. This replaces
upstream's git-submodule / CMake-time-download fallback, neither
of which is acceptable in OpenWrt's offline build model.

gcem is header-only (INTERFACE-only CMake target), so there is
no shared library to link against; the dependency exists at
build time only, expressed as PKG_BUILD_DEPENDS:=gcem.

Link: https://github.com/FluidSynth/fluidsynth/releases/tag/v2.5.4
Link: https://github.com/FluidSynth/fluidsynth/wiki/ReleaseNotes
Signed-off-by: Daniel Golle <redacted>