tests/audiogen: Fix total RIFF chunk size

The "RIFF" identifier and chunk size fields should not be included
within the size value.

tests/audiogen.c

@@ -109,7 +109,7 @@ static void put32(uint32_t v)
     fputc((v >> 24) & 0xff, outfile);
 }
 
-#define HEADER_SIZE      46
+#define HEADER_SIZE      38
 #define FMT_SIZE         18
 #define SAMPLE_SIZE       2
 #define WFORMAT_PCM  0x0001