st/nine: Fix compilation error on non-x86 platforms

The unused variable was improperly declared.

Closes: #4647
Fixes: d9e79bfe ("st/nine: Disable fpu exceptions during init")

Signed-off-by: Axel Davy <davyaxel0@gmail.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <!10263>
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