Add: support filtering content entries for patchpacks

This changes nothing for us, but allows patchpacks to add the
right pieces of code to start filtering content entries on patchpack
only entries.

(cherry picked from commit 77860bb61e61307e4d9cf869ba62be8ca4d9d9b3)

src/rev.cpp.in

@@ -79,6 +79,13 @@ const byte _openttd_revision_modified = ${REV_MODIFIED};
  */
 const byte _openttd_revision_tagged = ${REV_ISTAG};
 
+/**
+ * To check compatibility of BaNaNaS content, this version string is used.
+ * It should never contain things like "beta", but only the release version
+ * we are compatible with.
+ */
+const char _openttd_content_version[] = "12.0";
+
 /**
  * The NewGRF revision of OTTD:
  * bits  meaning.