Fix MicroBenchmark build on Linux with clang 18.1.8

[omjavaid]

MicroBenchmarks/libs/benchmark/test/options_test.cc fails to build on Linux/AArch64 with following error: error: variable 'actual_iterations' set but not used

This patch adds benchmark::DoNotOptimize(actual_iterations); to to function BM_explicit_iteration_count in options_test.cc

-Wall and -Werror were being used to compile and I am surprised that this was not caught by any of the buildbots. Some versions of clang compile this all fine with -Wall -Werror.