[C++] add +build ignore magic comment for go build

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>
diff --git a/regen.cc b/regen.cc
index 137d205..5b42d8f 100644
--- a/regen.cc
+++ b/regen.cc
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build ignore
+
 #include "regen.h"
 
 #include <sys/stat.h>