Include unistd.h for ssize_t

When compiling on buildroot I get:
`src/tls.c:235:28: error: ‘ssize_t’ undeclared (first use in this
function); did you mean ‘size_t’? `

To have this POSIX type available we will need to include
`sys/types.h` or `unistd.h`.

src/tls.c

@@ -21,6 +21,10 @@
 #include <stdarg.h>
 #include <string.h>
 
+#if !defined(_WIN32)
+#include <unistd.h>
+#endif
+
 #include "strophe.h"
 
 #include "common.h"