Fix warning about invalid identifier

A recent change to make the epfmi library unique caused the possiblity
of the library name starting with a numeric character, which triggers a
warning.

This change is to prefix the library name with "epfmi_"

ref lbl-srg/modelica-buildings#2956
ref lbl-srg/modelica-buildings#2220

lib/fmugenerator.cpp

@@ -45,7 +45,7 @@ void energyplusToFMU(const std::string &jsoninput,
     spawn_fs::create_directories(outputroot);
   }
 
-  const auto id = util::uniqueId();
+  const auto id = std::string("epfmi_") + util::uniqueId();
 
   const auto modelDescriptionPath = fmuStagingPath / "modelDescription.xml";
   const auto fmuResourcesPath = fmuStagingPath / "resources";