Fix libfbjs.

Summary:
Currently, I am unable to build libfbjs. More specifically, I get the following error:

g++  -fPIC -Wall -DNOT_FBMAKE=1 -ggdb -g -O0 -DDEBUG  -c -o parser.yacc.o parser.yacc.cpp
In file included from parser.hpp:35:0,
                 from parser.yy:23:
parser.yacc.hpp:186:31: error: ‘Node’ has not been declared
 int yyparse (void* yyscanner, Node* root);
                               ^
make: *** [parser.yacc.o] Error 1

It seems that the issue is that in the generated parser.yacc.hpp, Node isn't fully qualified with the correct namespace (i.e. libfbjs::Node). Comparing parser.y from libfbjs with the same file from xhpast, it seems that the namespace is missing from libfbjs.

Test Plan: After this change, I am able to build libfbjs using make.

Reviewers: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9446