Fix compilation on systems with older compilers.

This patch fixes a build error with GCC 7.x which doesn't seem to accept
const int's as constants in macro initialization.

See: tpo/core/tor#40410
9 jobs for ahf/bug40410_046 in 10 minutes and 50 seconds (queued for 2 seconds)
latest
Status Name Job ID Coverage
  Test
passed debian-disable-dirauth #24973

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passed debian-disable-relay #24974

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passed debian-distcheck #24969

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passed debian-docs #24970

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passed debian-hardened #24968

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passed debian-integration #24971

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passed debian-minimal #24967

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passed debian-nss #24975

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passed debian-tracing #24972

00:04:47