Fix data ordering bug causing problems on Debian Jessie

Got very odd results on Debian Jessie when having the uev_private_t last
in uev_t.  Turns out that when uev.c set "w->active = 0;" the w->fd data
member was written to instead.

Placing the private data first in uev_t is better practise, generally a
good thing for linked lists built on the BSD sys/queue.h API, looks a
lot better, but foremost works.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

uev.h

@@ -41,10 +41,12 @@
 
 /* Event watcher */
 typedef struct uev {
+	/* Private data for libuEv internal engine */
+	uev_private_t   type;
+
+	/* Public data for users to reference  */
 	int             fd;
 	uev_ctx_t      *ctx;
-
-	uev_private_t   type;
 } uev_t;
 
 /* Generic callback for watchers */