Fix the build on MacOS

The build on MacOS fails (https://github.com/google/nomulus/issues/67) due to different syntax for sed on BSD vs. Linux.

See this StackOverflow discussion: https://stackoverflow.com/questions/5694228/sed-in-place-flag-that-works-both-on-mac-bsd-and-linux

Also adds a newline between @SuppressWarnings annotation and the class definition.

Note that MacOS support is best-effort only.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161661181

java/google/registry/xjc/BUILD

@@ -44,9 +44,9 @@ genrule(
         "cp $(SRCS) $$TMP",
         "cd $$TMP",
         "$$XJC -npa -quiet -extension -d $$OUT -b *.xjb *.xsd",
-        ("find $$OUT -name '*.java' | xargs sed -i 's/" +
+        ("find $$OUT -name '*.java' | xargs sed -i.bak 's/" +
          "public class/" +
-         "@SuppressWarnings({\"unchecked\",\"serial\",\"cast\"}) " +
+         "@SuppressWarnings({\"unchecked\",\"serial\",\"cast\"})\\\n" +
          "public class/'"),
         "cd $$REP",
         "rm -rf $$TMP",