fix constraint violation in ftw

void* does not implicitly convert to function pointer types.

src/misc/ftw.c

@@ -3,7 +3,10 @@
 
 int ftw(const char *path, int (*fn)(const char *, const struct stat *, int), int fd_limit)
 {
-	return nftw(path, (void *)fn, fd_limit, FTW_PHYS);
+	/* The following cast assumes that calling a function with one
+	 * argument more than it needs behaves as expected. This is
+	 * actually undefined, but works on all real-world machines. */
+	return nftw(path, (int (*)())fn, fd_limit, FTW_PHYS);
 }
 
 LFS64(ftw);