Fix off-by-one in symbol next_id

Symbol last_id was changed to next_id, but it remained to be set to
tNEXT_ID - 1 initially, causing the initial static symbol to overlap with
the last built-in symbol in id.def.

symbol.c

@@ -99,7 +99,9 @@ typedef struct {
     VALUE ids;
 } rb_symbols_t;
 
-rb_symbols_t ruby_global_symbols = {tNEXT_ID-1};
+rb_symbols_t ruby_global_symbols = {
+    .next_id = tNEXT_ID,
+};
 
 struct sym_set_static_sym_entry {
     VALUE sym;