Mark decl of State::KeepRunningInternal() as inline

Maybe helps with
```
D:\a\_temp\msys64\ucrt64\bin\g++.exe -DHAVE_STD_REGEX -DHAVE_STEADY_CLOCK -DTEST_BENCHMARK_LIBRARY_HAS_NO_ASSERTIONS -ID:/a/benchmark/benchmark/include -Wall  -Wextra  -Wshadow  -Wfloat-equal  -Wold-style-cast  -Werror  -pedantic  -pedantic-errors  -fstrict-aliasing  -Wno-deprecated-declarations  -Wno-deprecated  -Wstrict-aliasing  -Wno-unused-variable -std=c++11 -fvisibility=hidden -fno-keep-inline-dllexport   -UNDEBUG -MD -MT test/CMakeFiles/benchmark_test.dir/benchmark_test.cc.obj -MF test\CMakeFiles\benchmark_test.dir\benchmark_test.cc.obj.d -o test/CMakeFiles/benchmark_test.dir/benchmark_test.cc.obj -c D:/a/benchmark/benchmark/test/benchmark_test.cc
In file included from D:/a/benchmark/benchmark/test/benchmark_test.cc:1:
D:/a/benchmark/benchmark/include/benchmark/benchmark.h:1007:37: error: 'bool benchmark::State::KeepRunningInternal(benchmark::IterationCount, bool)' redeclared without dllimport attribute after being referenced with dll linkage [-Werror]
 1007 | inline BENCHMARK_ALWAYS_INLINE bool State::KeepRunningInternal(IterationCount n,
      |                                     ^~~~~
```

include/benchmark/benchmark.h

@@ -982,7 +982,7 @@ class BENCHMARK_EXPORT State {
   void StartKeepRunning();
   // Implementation of KeepRunning() and KeepRunningBatch().
   // is_batch must be true unless n is 1.
-  bool KeepRunningInternal(IterationCount n, bool is_batch);
+  inline bool KeepRunningInternal(IterationCount n, bool is_batch);
   void FinishKeepRunning();
 
   const std::string name_;