[C] Fix empty argument function signatures.

Author: denizevrenci

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

@@ -480,7 +480,7 @@ private static CharSequence generateGroupPropertyFunctions(
  final String propertyName = formatPropertyName(token.name());
 
  sb.append(String.format("\n" +
- "SBE_ONE_DEF uint16_t %1$s_id()\n" +
+ "SBE_ONE_DEF uint16_t %1$s_id(void)\n" +
  "{\n" +
  " return %2$d;\n" +
  "}\n",
@@ -1801,7 +1801,7 @@ private CharSequence generateConstPropertyFunctions(
  containingStructName));
 
  sb.append(String.format("\n" +
- "SBE_ONE_DEF const char *%3$s_%1$s()\n" +
+ "SBE_ONE_DEF const char *%3$s_%1$s(void)\n" +
  "{\n" +
  " static uint8_t %1$s_values[] = {%2$s};\n\n" +