tools: Add a sidxindex tool

This tool can write an MPD file for fragmented MP4 files with
a sidx index at the start of the file.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/Makefile

@@ -412,3 +412,4 @@ TOOLS     = aviocat                                                     \
             ismindex                                                    \
             pktdumper                                                   \
             probetest                                                   \
+            sidxindex                                                   \