BUG/MINOR: checks/server: use_ssl member must be signed

(cherry picked from commit 4a8c026117f2f19bd2f8f0ac64954c8a4f717bc8)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>

[Cf: Forgot to mention to backport it with the commit 8a5bf35ce
     ("BUG/MINOR: checks: Respect the no-check-ssl option")]

(cherry picked from commit f1dc6a462c30355f4d886d9c5e3af48477177e63)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/include/types/server.h b/include/types/server.h
index 5edb273..99f2653 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -199,7 +199,7 @@
 	enum obj_type obj_type;                 /* object type == OBJ_TYPE_SERVER */
 	enum srv_state next_state, cur_state;   /* server state among SRV_ST_* */
 	enum srv_admin next_admin, cur_admin;   /* server maintenance status : SRV_ADMF_* */
-	char use_ssl;			        /* ssl enabled (1: on, 0: disabled, -1 forced off)  */
+	signed char use_ssl;		        /* ssl enabled (1: on, 0: disabled, -1 forced off)  */
 	unsigned int pp_opts;                   /* proxy protocol options (SRV_PP_*) */
 	struct server *next;
 	int cklen;				/* the len of the cookie, to speed up checks */