fix embedding of grammar with multiple_entries

bug reported by David Pierre here:
    http://article.gmane.org/gmane.comp.parsers.spirit.general/8860


[SVN r28010]

libs/spirit

@@ -1 +1 @@
-Subproject commit 3695252130e4e5a7e4d075ba86b828fee6afff0c
+Subproject commit b96fa9574461485d7bd9c189dccdf610cb9964d3