avutil: Include io.h with a separate condition from MapViewOfFile

The existence of MapViewOfFile isn't linked to the existence of
io.h.

Not all versions of windows have MapViewOfFile (in particular,
Windows Phone 8 and the "metro" windows 8 API subset don't),
while they still have io.h (and need it for open/read/close).
Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/file.c

@@ -25,10 +25,12 @@
 #if HAVE_UNISTD_H
 #include <unistd.h>
 #endif
+#if HAVE_IO_H
+#include <io.h>
+#endif
 #if HAVE_MMAP
 #include <sys/mman.h>
 #elif HAVE_MAPVIEWOFFILE
-#include <io.h>
 #include <windows.h>
 #endif