gom: Fix build with C++ compilers

By moving enum typedefs after the enum definition.

Putting the typedefs above the enum makes compilation fail for C++ programs. Example error:

/gom-filter.h:38:14: error: use of enum ‘_GomFilterMode’ without previous declaration
 typedef enum   _GomFilterMode    GomFilterMode;
               ^

https://bugzilla.gnome.org/show_bug.cgi?id=729831