CLEANUP: Add haproxy/xxhash.h to avoid modifying import/xxhash.h

This solves setting XXH_INLINE_ALL in a cleaner way, because the imported
header is not modified, easing future updates.

see 6f7cc11e6dd0f01b437fba893da2edd2362660a2
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index ae30c2c..a87d70b 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -43,7 +43,6 @@
 #include <import/ebpttree.h>
 #include <import/ebsttree.h>
 #include <import/lru.h>
-#include <import/xxhash.h>
 
 #include <haproxy/api.h>
 #include <haproxy/arg.h>
@@ -79,6 +78,7 @@
 #include <haproxy/tools.h>
 #include <haproxy/vars.h>
 #include <haproxy/xprt_quic.h>
+#include <haproxy/xxhash.h>
 
 
 /* ***** READ THIS before adding code here! *****