compiler/libec/grammars: Fixed sizeof("a" + 1)

compiler/libec/grammars: Fixed sizeof("a" + 1)
- This was previously causing a syntax error
- The sizeof rule had been split off in 2 rules in December 2005:
   SIZEOF '(' unary_expression ')'
   SIZEOF simple_unary_expression
  This was causing these errors. The reason for the split is long forgotten,
  perhaps this was to avoid conflicts which we now see as unavoidable
  for error tolerance with Bison.