src/parse/parser.ypp: use %empty token for empty rule

New `bison --warnings` gives the following helpful diagnostic:

      GEN      src/parse/parser.cc
    src/parse/parser.ypp:60.2: warning: empty rule without %empty [-Wempty-rule]
       60 | : /* empty */
          |  ^
          |  %empty

%empty was introduced in bison-2.7.90 around 2013.

src/parse/parser.ypp

@@ -57,7 +57,7 @@ extern "C" {
 %%
 
 spec
-: /* empty */
+: %empty
 | spec TOKEN_BLOCK {
     if (use_block(context, ast.temp_blockname) != Ret::OK) YYABORT;
     ast.temp_blockname.clear();