macosx: Initialise itemImages set with set correctly, not with an array

Fixes the following compile warnings:

/Users/claucambra/Downloads/GitRepos/vlc/extras/package/macosx/../../../modules/gui/macosx/library/VLCLibraryImageCache.m:179:45: warning: incompatible pointer types initializing 'NSMutableSet<NSImage *> *' with an expression of type 'NSMutableArray * _Nonnull' [-Wincompatible-pointer-types]
            NSMutableSet<NSImage *> * const itemImages = NSMutableArray.array;
                                            ^            ~~~~~~~~~~~~~~~~~~~~
/Users/claucambra/Downloads/GitRepos/vlc/extras/package/macosx/../../../modules/gui/macosx/library/VLCLibraryImageCache.m:191:70: warning: incompatible pointer types sending 'NSMutableSet<NSImage *> *' to parameter of type 'NSArray<NSImage *> *' [-Wincompatible-pointer-types]
                [NSImage framesForCompositeImageSquareGridWithImages:itemImages size:size gridItemCount:kVLCCompositeImageDefaultCompositedGridItemCount];
                                                                     ^~~~~~~~~~
/Users/claucambra/Downloads/GitRepos/vlc/extras/package/macosx/../../../modules/gui/macosx/extensions/NSImage+VLCAdditions.h:40:97: note: passing argument to parameter 'images' here
+ (NSArray<NSValue *> *)framesForCompositeImageSquareGridWithImages:(NSArray<NSImage *> * const)images
                                                                                                ^
/Users/claucambra/Downloads/GitRepos/vlc/extras/package/macosx/../../../modules/gui/macosx/library/VLCLibraryImageCache.m:193:51: warning: incompatible pointer types sending 'NSMutableSet<NSImage *> *' to parameter of type 'NSArray<NSImage *> *' [-Wincompatible-pointer-types]
                [NSImage compositeImageWithImages:itemImages frames:frames size:size];
                                                  ^~~~~~~~~~
/Users/claucambra/Downloads/GitRepos/vlc/extras/package/macosx/../../../modules/gui/macosx/extensions/NSImage+VLCAdditions.h:37:70: note: passing argument to parameter 'images' here
+ (instancetype)compositeImageWithImages:(NSArray<NSImage *> * const)images
                                                                     ^

Signed-off-by: Claudio Cambra developer@claudiocambra.com