Fixed warning: no previous extern declaration for non-static variable 'ResamplerFilter'

Fixes https://github.com/libsdl-org/SDL/issues/9556

src/audio/SDL_audioresample.c

@@ -447,7 +447,7 @@ static float Sinc(float *table, int x, int y)
     return (s * y) / x;
 }
 
-Cubic ResamplerFilter[RESAMPLER_SAMPLES_PER_ZERO_CROSSING][RESAMPLER_SAMPLES_PER_FRAME];
+static Cubic ResamplerFilter[RESAMPLER_SAMPLES_PER_ZERO_CROSSING][RESAMPLER_SAMPLES_PER_FRAME];
 
 static void GenerateResamplerFilter()
 {