xtensa: replace <asm-generic/export.h> with <linux/export.h>

Commit ddb5cdba ("kbuild: generate KSYMTAB entries by modpost")
deprecated <asm-generic/export.h>, which is now a wrapper of
<linux/export.h>.

Replace #include <asm-generic/export.h> with #include <linux/export.h>.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20231126151222.1556761-1-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>