stringtable.d: add some const

[WalterBright]

No description provided.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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[wilzbach]

SemaphoreCI failed with:

Test compilable/teststdint.d failed.  The logged output:
../generated/linux/release/64/dmd -conf= -m64 -Icompilable  -fPIC  -odtest_results/compilable -oftest_results/compilable/teststdint_0.o  -c compilable/teststdint.d 
compilable/teststdint.d(211): Error: static assert:  `"_Z10int_fast16i" == "_Z10int_fast16l"` is false

Looks spurious though. Restarted.

[wilzbach]

This was introduced in #8252