Don't use stdin as an lvalue

freopen will always returns its last argument or 0. Therefore there
is no need of the unportable pratice of assigning stdin a value.

This fixes compilation with musl libc

fdupes.c

@@ -1238,7 +1238,7 @@ int main(int argc, char **argv) {
     }
     else
     {
-      stdin = freopen("/dev/tty", "r", stdin);
+      freopen("/dev/tty", "r", stdin);
       deletefiles(files, 1, stdin);
     }
   }