Fix mac build, make pipe_stats linux-only

Follow-up to a previous CL that introduced pipestats.
It doesn't work on Mac and it is useful only on Linux
host anyways. Surrounding its block with conditionals.

Bug: 76169489
Change-Id: I6cd920bb0dbe1507d54e082403f6cc1ab81ff62c
diff --git a/BUILD.gn b/BUILD.gn
index a5d896e..ca0943d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -48,6 +48,9 @@
     if (is_linux || is_android) {
       deps += [ "tools/skippy" ]
     }
+    if (is_linux) {
+      deps += [ "tools:pipestats" ]
+    }
   }
 }
 
@@ -59,11 +62,10 @@
     "src/base:unittests",
     "src/protozero:unittests",
   ]
+
   # TODO(brucedawson): Enable these for Windows when possible.
   if (!is_win) {
-    deps += [
-      "src/tracing:unittests",
-    ]
+    deps += [ "src/tracing:unittests" ]
   }
   if (!build_with_chromium) {
     deps += [