don't include os_compat.h

When there are standardized headers, use these instead of the bloated
os_compat.h.

src/dirvec.c

@@ -1,8 +1,9 @@
 #include "dirvec.h"
 #include "directory.h"
-#include "os_compat.h"
 #include "utils.h"
 
+#include <string.h>
+
 static size_t dv_size(struct dirvec *dv)
 {
 	return dv->nr * sizeof(struct directory *);