build: [gn] depend on /gtk instead of /gtk3

(We still depend on gtk3, just via one extra layer of abstraction. The
/gtk target itself depends on /gtk3.)

BUILD.gn

@@ -254,7 +254,7 @@ static_library("electron_lib") {
   }
   if (is_linux) {
     deps += [
-      "//build/config/linux/gtk3",
+      "//build/config/linux/gtk",
       "//chrome/browser/ui/libgtkui",
       "//device/bluetooth",
       "//ui/events/devices/x11",

brightray/BUILD.gn

@@ -51,7 +51,7 @@ static_library("brightray") {
 
   if (is_linux) {
     deps += [
-      "//build/config/linux/gtk3",
+      "//build/config/linux/gtk",
       ":libnotify",
     ]
   }