macosx: prefs: fix function prototype

The function is actually taking a vlc_config_subcat both from its definition and its usage, which was creating warnings:

modules/gui/macosx/preferences/prefs.m:533:82: warning: implicit conversion from enumeration type 'enum vlc_config_subcat' to different enumeration type 'enum vlc_config_cat' [-Wenum-conversion]
                    subcategoryItem = [categoryItem childRepresentingSubcategory:subcat];
                                      ~                                          ^~~~~~
modules/gui/macosx/preferences/prefs.m:675:77: warning: conflicting parameter types in implementation of 'childRepresentingSubcategory:': 'enum vlc_config_cat' vs 'enum vlc_config_subcat' [-Wmismatched-parameter-types]
- (VLCTreeBranchItem *)childRepresentingSubcategory:(enum vlc_config_subcat)subcategory
                                                     ~~~~~~~~~~~~~~~~~~~~~~ ^
modules/gui/macosx/preferences/prefs.m:128:74: note: previous definition is here
- (VLCTreeBranchItem *)childRepresentingSubcategory:(enum vlc_config_cat)category;
                                                     ~~~~~~~~~~~~~~~~~~~ ^