fluidsynth: Use a signed atomic type to avoid warnings.

libs/fluidsynth/src/synth/fluid_synth.h

@@ -120,7 +120,7 @@ struct _fluid_synth_t
     int effects_channels;              /**< the number of effects channels (>= 2) */
     int effects_groups;                /**< the number of effects units (>= 1) */
     int state;                         /**< the synthesizer state */
-    fluid_atomic_uint_t ticks_since_start;    /**< the number of audio samples since the start */
+    fluid_atomic_int_t ticks_since_start;    /**< the number of audio samples since the start */
     unsigned int start;                /**< the start in msec, as returned by system clock */
     fluid_overflow_prio_t overflow;    /**< parameters for overflow priority (aka voice-stealing) */