[C] Add missing storage class specifier.

Author: denizevrenci

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/c/CGenerator.java

@@ -2282,7 +2282,7 @@ private static void generateCompositePropertyFunction(
         final int offset = token.offset();
 
         sb.append(String.format("\n" +
-            "struct %1$s *%4$s_%2$s(struct %4$s *const codec, struct %1$s *const composite)\n" +
+            "SBE_ONE_DEF struct %1$s *%4$s_%2$s(struct %4$s *const codec, struct %1$s *const composite)\n" +
             "{\n" +
             "    return %1$s_wrap(\n" +
             "        composite,\n" +