Cast type explicitly for fix build error

[neoatlan]

Issue #, if available:
Fix build error on Apple clang version 12.0.0 (clang-1200.0.32.27)

src/backward.h:3265:57: error: 
      implicit conversion loses integer precision: 'size_t'
      (aka 'unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
        _symbols.reset(backtrace_symbols(st.begin(), st.size()));

Description of changes:
Cast type explicitly

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[bmoffatt]

Thanks!