grub2: fix compile error with glibc 2.19.1

BUG=chromium:400426
TEST=grub compiles with glibc 2.19.1

Change-Id: Icef6829461ab037f356b66720f788617ac165da1
Reviewed-on: https://chromium-review.googlesource.com/211001
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
diff --git a/grub-core/gnulib/stdio.in.h b/grub-core/gnulib/stdio.in.h
index 80b9dbf..9147bde 100644
--- a/grub-core/gnulib/stdio.in.h
+++ b/grub-core/gnulib/stdio.in.h
@@ -140,8 +140,10 @@
 /* It is very rare that the developer ever has full control of stdin,
    so any use of gets warrants an unconditional warning.  Assume it is
    always declared, since it is required by C89.  */
+#if defined gets
 #undef gets
 _GL_WARN_ON_USE (gets, "gets is a security hole - use fgets instead");
+#endif
 
 #if @GNULIB_FOPEN@
 # if @REPLACE_FOPEN@