BUILD: http: include types/sample.h in proto_http.h

Commit d7c9196 ("MAJOR: filters: Add filters support") removed sample.h
from proto_http.h, but it has become necessary as of commit fd7edd3
("MINOR: Move http method enum from proto_http to sample") in order
to have HTTP_METH_*. Due to this, the "debug/flags" utility doesn't
build anymore.
diff --git a/include/types/proto_http.h b/include/types/proto_http.h
index 935be8c..0c1f50f 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -29,6 +29,7 @@
 
 #include <types/hdr_idx.h>
 #include <types/filters.h>
+#include <types/sample.h>
 
 /* These are the flags that are found in txn->flags */