detect/flowbits: align all pointers in the struct

Makes leak sanitizer work without adding LSAN_OPTIONS=use_unaligned=1

Otherwise, leak sanitizer may report the read and black tree pointers
as leaked when it is still owned by some global variable.

Follow-up on d046e82db6

src/detect-flowbits.c

@@ -1034,10 +1034,10 @@ static bool PrefilterFlowbitIsPrefilterable(const Signature *s)
  * found. Part of a rb-tree. */
 typedef struct PrefilterFlowbit {
     uint32_t *rule_id;     /**< array of signature iid that are part of this prefilter */
+    RB_ENTRY(PrefilterFlowbit) __attribute__((__packed__)) rb;
     uint32_t id;           /**< flowbit id */
     uint32_t rule_id_size; /**< size in elements of `rule_id` */
     uint32_t rule_id_cnt;  /**< usage in elements of `rule_id` */
-    RB_ENTRY(PrefilterFlowbit) __attribute__((__packed__)) rb;
 } __attribute__((__packed__)) PrefilterFlowbit;
 
 static int PrefilterFlowbitCompare(const PrefilterFlowbit *a, const PrefilterFlowbit *b)