Fix dll linkage on mingw64

[kingscallop]

On mingw64, functions that have a dllimport attribute and are inline functions, need to have the inline specifier on both the definition and declaration when they are present in the same header file. Otherwise the following warning appears:

spark/include/Extensions/Modifiers/SPK_EmitterAttacher.h:132:36: warning: 'const SPK::Ref<SPK::Emitter>& SPK::EmitterAttacher::getEmitter() const' redeclared without dllimport attribute after being referenced with dll linkage
  132 |         inline const Ref<Emitter>& EmitterAttacher::getEmitter() const
      |                                    ^~~~~~~~~~~~~~~