GH-1876 turn pack200 into a shared library

libraries/pack200/include/unpack200.h

@@ -24,7 +24,8 @@
  */
 
 #pragma once
-#include <string>
+
+#include "multimc_unpack200_export.h"
 
 /**
  * @brief Unpack a PACK200 file
@@ -33,4 +34,4 @@
  * @param output_path Path to the output file in PACK200 format. System native string encoding.
  * @throw std::runtime_error for any error encountered
  */
-void unpack_200(FILE * input_path, FILE * output_path);
+MULTIMC_UNPACK200_EXPORT void unpack_200(FILE * input_path, FILE * output_path);