avutil/adler32: Fix data type in test code

Fixes "warning: argument #2 is incompatible with prototype:"
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/adler32.c

@@ -108,7 +108,7 @@ static volatile int checksum;
 int main(int argc, char **argv)
 {
     int i;
-    char data[LEN];
+    uint8_t data[LEN];
 
     av_log_set_level(AV_LOG_DEBUG);