Linux compat: fix DECLARE_EVENT_CLASS() test when ZFS is built-in

ZFS_LINUX_TRY_COMPILE_HEADER macro doesn't take CONFIG_ZFS=y into
account. As a result, on several latest Linux versions, configure
script marks DECLARE_EVENT_CLASS() available for non-GPL when ZFS
is being built as a module, but marks it unavailable when ZFS is
built-in.
Follow the logic of the neighbor macros and adjust
ZFS_LINUX_TRY_COMPILE_HEADER accordingly, so that it doesn't try
to look for a .ko when ZFS is built-in.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Closes #14006