BUILD: haring: fix compile issue due to nop warning.

Compile issue introduced by commit c0b39ea75e ("DEV: haring: bound the
ring geometry to the size of the file").

This patch introduce a warning at compile the binary is not
compiling in case of -Wno-error because of statement with no effects.

In addition it seems also an error since the added static var was never set.

It seems to use a static to rise an error message only one time. So I
suppose it should be set to 1 as soon the error message is printed.

This patch must backported until 3.4.

dev/haring/haring.c

@@ -176,7 +176,7 @@ int dump_ring_as_buf(struct buffer buf, size_t ofs, int flags)
 					fprintf(stderr, "Note: incomplete message at offset %llu, dumping "
 						"its first %llu byte(s) and stopping.\n",
 						(unsigned long long)ofs, (unsigned long long)msg_len);
-					warned;
+					warned = 1;
 				}
 				truncated = 1;
 			}