OpenBSD compilation fix

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

src/config/file.c

@@ -34,6 +34,7 @@
 #include <assert.h>
 #include <limits.h>
 #include <fcntl.h>
+#include <sys/stat.h>
 #ifdef __APPLE__
 #   include <xlocale.h>
 #else