[llvm] [GSYM] Silence cast warning (PR #191561)

[Roy Shi via llvm-commits]

royitaqi wrote:

Do we still need this after https://github.com/llvm/llvm-project/pull/189524 merged? IIUC, the `StrtabSize` was changed to `uint64_t`, so this warning should have disappeared already.

https://github.com/llvm/llvm-project/pull/191561