Make the demo directory check more robust

This is how it should've been to begin with, but I was lazy.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/os.h b/src/os.h
index c00eaf3..3d3ab00 100644
--- a/src/os.h
+++ b/src/os.h
@@ -46,6 +46,7 @@
 #include <string.h>
 #include <strings.h>
 #include <sys/mman.h>
+#include <sys/stat.h>
 #include <unistd.h>
 #endif