Fix build errors related to template instantiations on Windows

The MS compiler doesn't seem to recognize our templates here:

g:\jenkins\workspace\simgear-win\source\simgear\embedded_resources\EmbeddedResourceManager.hxx(169): error C3190: 'std::shared_ptr<const simgear::AbstractEmbeddedResource> simgear::EmbeddedResourceManager::getResource(const std::string &,const std::string &) const' with the provided template arguments is not the explicit instantiation of any member function of 'simgear::EmbeddedResourceManager' (compiling source file G:\Jenkins\workspace\SimGear-Win\source\simgear\embedded_resources\EmbeddedResourceManager.cxx) [G:\Jenkins\workspace\SimGear-Win\build32\simgear\SimGearCore.vcxproj]
g:\jenkins\workspace\simgear-win\source\simgear\embedded_resources\EmbeddedResourceManager.hxx(169): error C2945: explicit instantiation does not refer to a template-class specialization (compiling source file G:\Jenkins\workspace\SimGear-Win\source\simgear\embedded_resources\EmbeddedResourceManager.cxx) [G:\Jenkins\workspace\SimGear-Win\build32\simgear\SimGearCore.vcxproj]

-> try without the explicit instantiations on Windows.

simgear/embedded_resources/EmbeddedResourceManager.hxx

@@ -163,6 +163,11 @@ private:
 };
 
 // Explicit template instantiations
+//
+// MSVC doesn't recognize these as template instantiations of what we defined
+// above (this seems to be with “Visual Studio 14 2015”), therefore I'm not
+// including them on Windows.
+#ifndef _WIN32
 template
 std::shared_ptr<const AbstractEmbeddedResource>
 EmbeddedResourceManager::getResource(const std::string& virtualPath,
@@ -194,6 +199,7 @@ EmbeddedResourceManager::getIStream(const std::string& virtualPath,
 template
 std::unique_ptr<std::istream>
 EmbeddedResourceManager::getIStream(const std::string& virtualPath) const;
+#endif  // ifndef _WIN32
 
 } // of namespace simgear