iconCache: Gio.Icon.equal(), not .equals()

[ferdnyc]

The comparison function is named equal(). https://docs.gtk.org/gio/method.Icon.equal.html

Fixes this traceback:

Apr 14 17:15:41 gnome-shell[2368055]: 
remmina-icon unable to update icon: TypeError: oldIcon.equals is not a function
Stack trace:
add@file:///.../appindicatorsupport@rgcjonas.gmail.com/iconCache.js:52:34
_cacheOrCreateIconByName@file:///.../appindicatorsupport@rgcjonas.gmail.com/appIndicator.js:1074:37
async*_createIcon@file:///.../appindicatorsupport@rgcjonas.gmail.com/appIndicator.js:1440:38
_createAndSetIcon@file:///.../appindicatorsupport@rgcjonas.gmail.com/appIndicator.js:1400:32
_updateIconByType@file:///.../appindicatorsupport@rgcjonas.gmail.com/appIndicator.js:1392:24
_updateIcon@file:///.../appindicatorsupport@rgcjonas.gmail.com/appIndicator.js:1469:24
_invalidateIcon@file:///.../appindicatorsupport@rgcjonas.gmail.com/appIndicator.js:1524:14
_invalidateIconWhenFullyReady@file:///.../appindicatorsupport@rgcjonas.gmail.com/appIndicator.js:1507:18
async*_init/<@file:///.../appindicatorsupport@rgcjonas.gmail.com/appIndicator.js:915:24
_callHandlers@resource:///org/gnome/gjs/modules/core/_signals.js:130:42
_emit@resource:///org/gnome/gjs/modules/core/_signals.js:119:10
reset@file:///.../appindicatorsupport@rgcjonas.gmail.com/appIndicator.js:698:14
_ensureItemRegistered@file:///.../appindicatorsupport@rgcjonas.gmail.com/statusNotifierWatcher.js:136:18
RegisterStatusNotifierItemAsync@file:///.../appindicatorsupport@rgcjonas.gmail.com/statusNotifierWatcher.js:205:24
_handleMethodCall@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:373:35
_wrapJSObject/<@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:408:34
@resource:///org/gnome/shell/ui/init.js:21:20

[ferdnyc]

Er, sorry. Wrong reference, right fix: https://docs.gtk.org/gio/vfunc.Icon.equal.html

(Edit: Fixed commit message.)