Fix build of all-clusters-app to not build extraneous things. (#21302)

The all-clusters-app .gn file did not say what the default thing to
build is, so we were defaulting to "all targets declared in any .gn
file we depend on".

This led to us compiling address-resolve-tool, for example, in
addition to all-clusters-app.

The fix is to explicitly say what the default thing to build is.

Fixes #21221

examples/all-clusters-app/linux/BUILD.gn

@@ -121,3 +121,7 @@ group("linux") {
     deps += [ ":chip-all-clusters-app" ]
   }
 }
+
+group("default") {
+  deps = [ ":linux" ]
+}

examples/all-clusters-minimal-app/linux/BUILD.gn

@@ -101,3 +101,7 @@ group("linux") {
   deps = []
   deps += [ ":chip-all-clusters-minimal-app" ]
 }
+
+group("default") {
+  deps = [ ":linux" ]
+}