gcapi_mac: Add gn build file.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index f9c6907362d928823d7e44ed3f7ff07361d57c49..a310f481385ddbf4b27050e4c5ade5ab3cd57751 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -257,6 +257,10 @@ group("gn_all") {
     ]
   }
 
+  if (is_mac) {
+    deps += [ "//chrome/installer/gcapi_mac:gcapi_example" ]
+  }
+
   if (is_android) {
     deps += [
       "//base:base_junit_tests",