Add LIBPBD_API to a function declaration

The new specialisation for ConfigVariable<float>::set_from_string() needs to be exportable (it gets used somehow by ARDOUR::SessionConfiguration).

If adding LIBPBD_API causes a problem for gcc, we could change it to LIBPBD_TEMPLATE_MEMBER_API

libs/pbd/pbd/configuration_variable.h

@@ -90,7 +90,7 @@ class /*LIBPBD_API*/ ConfigVariable : public ConfigVariableBase
 };
 
 /** Specialisation of ConfigVariable to deal with float (-inf etc) */
-template<> void
+template<> LIBPBD_API void
 ConfigVariable<float>::set_from_string (std::string const & s);
 
 /** Specialisation of ConfigVariable for std::string to cope with whitespace properly */