BUILD/MINOR: http: proto_http.h needs sample.h

Since commit fd7edd3 ("MINOR: Move http method enum from proto_http to sample")
proto_http.h needs to include sample.h. This can be backported to 1.6 though
it doesn't affect existing code.

include/types/proto_http.h

@@ -28,6 +28,7 @@
 #include <common/regex.h>
 
 #include <types/hdr_idx.h>
+#include <types/sample.h>
 
 /* These are the flags that are found in txn->flags */