ismindex: Fix build on mingw

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 8801fac3)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

tools/ismindex.c

@@ -35,6 +35,10 @@
 #include <stdio.h>
 #include <string.h>
 #include <sys/stat.h>
+#ifdef _WIN32
+#include <io.h>
+#define mkdir(a, b) mkdir(a)
+#endif
 
 #include "libavformat/avformat.h"
 #include "libavutil/intreadwrite.h"