Add export to << friend declaration for MSVC

Author: jeremy-rifkin

include/cpptrace/basic.hpp

@@ -166,7 +166,7 @@ namespace cpptrace {
 
         std::string to_string() const;
         std::string to_string(bool color) const;
-        friend std::ostream& operator<<(std::ostream& stream, const stacktrace_frame& frame);
+        friend CPPTRACE_EXPORT std::ostream& operator<<(std::ostream& stream, const stacktrace_frame& frame);
     };
 
     struct CPPTRACE_EXPORT stacktrace {
@@ -182,7 +182,7 @@ namespace cpptrace {
         void clear();
         bool empty() const noexcept;
         std::string to_string(bool color = false) const;
-        friend std::ostream& operator<<(std::ostream& stream, const stacktrace& trace);
+        friend CPPTRACE_EXPORT std::ostream& operator<<(std::ostream& stream, const stacktrace& trace);
 
         using iterator = std::vector<stacktrace_frame>::iterator;
         using const_iterator = std::vector<stacktrace_frame>::const_iterator;