Make -DMINEFIELD show up in Windows buildinfo.

I listed a lot of other build options, but not that one. The release
checklist still recommends doing test builds with it, so it seems
sensible to arrange that you can tell if a build _is_ one of those or
not.

misc.c

@@ -1214,6 +1214,9 @@ char *buildinfo(const char *newline)
     }
 #endif
 
+#if defined _WINDOWS && defined MINEFIELD
+    strbuf_catf(buf, "%sBuild option: MINEFIELD", newline);
+#endif
 #ifdef NO_SECURITY
     strbuf_catf(buf, "%sBuild option: NO_SECURITY", newline);
 #endif