Hide declaration of gets(3) for C11 and later.

gets(3) was removed from C11.

Change-Id: I4040a8a6c6d5d2a86878e8a80ca67964fbbb82a8

libstdc++/include/cstdio

@@ -63,7 +63,9 @@ using ::ftell;
 using ::fwrite;
 using ::getc;
 using ::getchar;
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ < 201112L
 using ::gets;
+#endif
 using ::perror;
 using ::printf;
 using ::putc;