blkg_tryget config test: initialize struct

Missing struct initialization in a config test results in the
interface being incorrectly detected.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Adam Moss <[email protected]>
Signed-off-by: Mathieu Velten <[email protected]>
Closes #10713 
Closes #11049

config/kernel-bio.m4

@@ -344,7 +344,7 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BLKG_TRYGET], [
 		#include <linux/bio.h>
 		#include <linux/fs.h>
 	],[
-		struct blkcg_gq blkg __attribute__ ((unused));
+		struct blkcg_gq blkg __attribute__ ((unused)) = {};
 		bool rc __attribute__ ((unused));
 		rc = blkg_tryget(&blkg);
 	], [], [$ZFS_META_LICENSE])