ssize_t comes from sys/types.h therefore include it

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>
--- a/wsrep-lib/include/wsrep/gtid.hpp
+++ b/wsrep-lib/include/wsrep/gtid.hpp
@@ -25,7 +25,7 @@
 #include "compiler.hpp"
 
 #include <iosfwd>
-
+#include <sys/types.h>
 /**
  * Minimum number of bytes guaratneed to store GTID string representation,
  * terminating '\0' not included (36 + 1 + 20).