fixed clang compile issue

src/ACGL/OpenGL/Data/TextureDataLoadStore.cc

@@ -326,7 +326,7 @@ std::istream& loadTextureDataFromPNM_filterPNMComments(std::istream& _in)
 
 SharedTextureData loadTextureDataFromPNM(const std::string &_filename)
 {
-    std::ifstream fileStream( _filename, std::ifstream::in );
+    std::ifstream fileStream( _filename.c_str() );
 
     if (!fileStream.good()) {
         error() << "could not open file " << _filename << std::endl;