Fix macro name for io_uring_queue_init_mem check.

Fix macro name for io_uring_queue_init_mem check.

Commit f54af9f2679d added a check for
io_uring_queue_init_mem(). However, it used the macro name
HAVE_LIBURING_QUEUE_INIT_MEM in both meson.build and the C code, while
the Autotools build script defined HAVE_IO_URING_QUEUE_INIT_MEM. As a
result, the optimization was never enabled in builds configured with
Autotools, as the C code checked for the wrong macro name.

This commit changes the macro name to HAVE_IO_URING_QUEUE_INIT_MEM in
meson.build and the C code. This matches the actual function
name (io_uring_queue_init_mem), following the standard HAVE_<FUNCTION>
convention.

Backpatch to 18, where the macro was introduced.

Bug: #19368
Reported-by: Evan Si <evsi@amazon.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19368-016d79a7f3a1c599@postgresql.org
Backpatch-through: 18