AVRO-3954: [C] Fix type in quickstop example

This removes the following warning:

  avro/lang/c/examples/quickstop.c:123:40: warning: incompatible pointer types passing 'int32_t **' (aka 'int **') to parameter of type
        'const char **' [-Wincompatible-pointer-types]
                          avro_value_get_string(&first_value, &p, &size);

Signed-off-by: Sahil Kang <[email protected]>
Signed-off-by: Sahil Kang <[email protected]>

lang/c/examples/quickstop.c

@@ -107,7 +107,7 @@ int print_person(avro_file_reader_t db, avro_schema_t reader_schema)
 	if (rval == 0) {
 		int64_t id;
 		int32_t age;
-		int32_t *p;
+		const char *p;
 		size_t size;
 		avro_value_t id_value;
 		avro_value_t first_value;