REORG: include: split common/http-hdr.h into haproxy/http-hdr{,-t}.h

There's only one struct and 2 inline functions. It could have been
merged into http.h but that would have added a massive dependency on
the hpack parts for nothing, so better keep it this way since hpack
is already freestanding and portable.
diff --git a/src/h1.c b/src/h1.c
index f30e489..f143db9 100644
--- a/src/h1.c
+++ b/src/h1.c
@@ -13,7 +13,7 @@
 #include <ctype.h>
 #include <haproxy/api.h>
 #include <common/h1.h>
-#include <common/http-hdr.h>
+#include <haproxy/http-hdr.h>
 
 #include <proto/channel.h>