MINOR: build: fix build dependency

fix include dependency. The header file sample.h don't need to known
the content of the struct arg, so I remove the include, and replace
it by a simple pointer declaration.

This prevent an include dependecy issue with the next patch.

include/types/sample.h

@@ -28,9 +28,10 @@
 
 #include <common/chunk.h>
 #include <common/mini-clist.h>
-#include <types/arg.h>
 #include <types/proto_http.h>
 
+struct arg;
+
 /* input and output sample types */
 enum {
 	SMP_T_ANY = 0,   /* any type */