test(stacktrace): make CPPTRACE_FORCE_INLINE static (#196)

This fixes compilation error:

```
> error: inlining failed in call to 'always_inline' 'int stacktrace_inline_resolution_2(std::vector<int>&)': function body can be overwritten at link time
```

Author: xokdvium

test/unit/stacktrace.cpp

@@ -11,7 +11,7 @@ using namespace std::literals;
 #ifdef _MSC_VER
  #define CPPTRACE_FORCE_INLINE [[msvc::flatten]]
 #else
- #define CPPTRACE_FORCE_INLINE [[gnu::always_inline]]
+ #define CPPTRACE_FORCE_INLINE [[gnu::always_inline]] static
 #endif