Fixed find_last_of call in GetBaseFilename

this leads to a stack overflow on linux - it looks like this magically works on msvc though...

tiny_gltf.h

@@ -2041,8 +2041,7 @@ static std::string GetBaseDir(const std::string &filepath) {
 }
 
 static std::string GetBaseFilename(const std::string &filepath) {
-  constexpr char path_separators[2] = { '/', '\\' };
-  auto idx = filepath.find_last_of(path_separators);
+  auto idx = filepath.find_last_of("/\\");
   if (idx != std::string::npos)
     return filepath.substr(idx + 1);
   return filepath;