Fix for compiling peg.d with keywords TRIE

Since 2.061 the trie form of the keywords rule can now be compiled with inlining turned on. This is a small fix to make peg.d compile with the current grammar.d. The speed gain with the trie is probably not worth the much longer compile time, but it is a little faster (~10%) at running my grammar. 

Once the new std.uni comes in, there might be a phobos trie implementation to try out.