Use fstream from boost::nowide

Fixes build on Boost 1.79.0.

src/libslic3r/utils.cpp

@@ -1405,7 +1405,7 @@ bool bbl_calc_md5(std::string &filename, std::string &md5_out)
     unsigned char digest[16];
     MD5_CTX       ctx;
     MD5_Init(&ctx);
-    boost::filesystem::ifstream ifs(filename, std::ios::binary);
+    boost::nowide::ifstream ifs(filename, std::ios::binary);
     std::string                 buf(64 * 1024, 0);
     const std::size_t &         size      = boost::filesystem::file_size(filename);
     std::size_t                 left_size = size;