When the implementation of function_call_header and
function_identifier were changed a few commits ago, the types of the
production changed. This just updates the types specified for the
productions to match reality.
%type <expression> function_call
%type <n> assignment_operator
%type <n> unary_operator
-%type <node> function_identifier
+%type <expression> function_identifier
%type <node> external_declaration
%type <declarator_list> init_declarator_list
%type <declarator_list> single_declaration