pgsql: Suppress unused-variable warning.

Suppress unused-variable warning.

x86 machines lacking HAVE__CPUIDEX saw a complaint about
"unused variable 'reg'", per buildfarm as well as local
experience.  Oversight in bcb2cf41f.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b268928f932cdfb899e96ab28badfe716298483e

Modified Files
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src/port/pg_cpu_x86.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)