[C] Fix bitset property accessor function storage-class.

Author: denizevrenci

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

@@ -2212,7 +2212,7 @@ private static void generateBitsetPropertyFunctions(
  final int offset = token.offset();
 
  sb.append(String.format("\n" +
- "struct %1$s *%4$s_%2$s(struct %4$s *const codec, struct %1$s *const bitset)\n" +
+ "SBE_ONE_DEF struct %1$s *%4$s_%2$s(struct %4$s *const codec, struct %1$s *const bitset)\n" +
  "{\n" +
  " return %1$s_wrap(\n" +
  " bitset,\n" +