configure: fix blkdev.h include for discard definition

A previous commit added compat support for this, but it's outputting
to the wrong file for the case where just blkdev.h is needed.

Fixes: 906a45673123 ("Add io_uring_prep_cmd_discard")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

configure

diff --git a/configure b/configure
index 6c9cea0..4794306 100755
--- a/configure
+++ b/configure
@@ -677,7 +677,7 @@
 #endif
 
 EOF
-else cat >> $discard_cmd << EOF
+else cat >> $compat_h << EOF
 #include <linux/blkdev.h>
 
 EOF