Update srs_app_hls.cpp

for hls, srs should not allow player to cache m3u8 list.

add #EXT-X-ALLOW-CACHE:NO to m3u8 file.

@@ -825,7 +825,10 @@ int SrsHlsMuxer::_refresh_m3u8(int& fd, string m3u8_file)
         0x23, 0x45, 0x58, 0x54, 0x4d, 0x33, 0x55, 0xa, 
         // #EXT-X-VERSION:3\n
         0x23, 0x45, 0x58, 0x54, 0x2d, 0x58, 0x2d, 0x56, 0x45, 0x52, 
-        0x53, 0x49, 0x4f, 0x4e, 0x3a, 0x33, 0xa
+        0x53, 0x49, 0x4f, 0x4e, 0x3a, 0x33, 0xa,
+        // #EXT-X-ALLOW-CACHE:NO
+        0x23, 0x45, 0x58, 0x54, 0x2d, 0x58, 0x2d, 0x41, 0x4c, 0x4c, 
+        0x4f, 0x57, 0x2d, 0x43, 0x41, 0x43, 0x48, 0x45, 0x3a, 0x4e, 0x4f, 0x0a
     };
     if (::write(fd, header, sizeof(header)) != sizeof(header)) {
         ret = ERROR_HLS_WRITE_FAILED;