mariadb: use liburing if kernel supports it

mariadb: use liburing if kernel supports it

liburing is preferred over libaio.  So, if liburing has already been
built, mariadb will use it, instead of libaio, failing with a missed
dependency.  If it is not found, then libaio is used.

Since it depends on the kernel being built with io_uring support, and it
defaults to y unless SMALL_FLASH=y, then we should allow libaio as
fallback, checking the CONFIG_KERNEL_IO_URING symbol.

Signed-off-by: Eneas U de Queiroz <redacted>