config/output: unbreak on 32-bit architectures

sway/config/output.c:624:54: error: format specifies type 'long' but the argument has type 'size_t' (aka 'unsigned int') [-Werror,-Wformat]
                sway_log(SWAY_DEBUG, "spawn_swaybg cmd[%ld] = %s", k, cmd[k]);
                                                       ~~~         ^
                                                       %zu
include/log.h:40:74: note: expanded from macro 'sway_log'
        _sway_log(verb, "[%s:%d] " fmt, _sway_strip_path(__FILE__), __LINE__, ##__VA_ARGS__)
                                                                                ^~~~~~~~~~~

sway/config/output.c

@@ -621,7 +621,7 @@ bool spawn_swaybg(void) {
 	}
 
 	for (size_t k = 0; k < i; k++) {
-		sway_log(SWAY_DEBUG, "spawn_swaybg cmd[%ld] = %s", k, cmd[k]);
+		sway_log(SWAY_DEBUG, "spawn_swaybg cmd[%zd] = %s", k, cmd[k]);
 	}
 
 	bool result = _spawn_swaybg(cmd);