generic: fix ramips-mt76x8 compile

The ramips-mt76x8 target is now flagged as a small-flash target. Those
targets can not be compiled with KERNEL_IO_URING support, thus the
config option is not exposed.

Try disabling the config option to avoid failing on this platform.

Signed-off-by: David Bauer <mail@david-bauer.net>

targets/generic

@@ -59,7 +59,7 @@ try_config('TARGET_SQUASHFS_BLOCK_SIZE', 256)
 
 config('KERNEL_PROC_STRIPPED', true)
 config('KERNEL_AIO', false)
-config('KERNEL_IO_URING', false)
+try_config('KERNEL_IO_URING', false)
 config('KERNEL_FHANDLE', false)
 config('KERNEL_FANOTIFY', false)
 config('KERNEL_CGROUPS', false)