std.os.linux: add explicit backing type to packed structs in extern types

lib/std/c/illumos.zig

@@ -297,7 +297,7 @@ pub const lifreq = extern struct {
         /// Maximum transmission unit
         mtu: u32,
         // Technically [2]i32
-        muxid: packed struct { ip: i32, arp: i32 },
+        muxid: packed struct(u64) { ip: i32, arp: i32 },
         /// Neighbor reachability determination entries
         nd_req: lif_nd_req,
         /// Link info

lib/std/os/linux.zig

@@ -9515,7 +9515,7 @@ pub const perf_event_attr = extern struct {
     sample_type: u64 = 0,
     read_format: u64 = 0,
 
-    flags: packed struct {
+    flags: packed struct(u64) {
         /// off by default
         disabled: bool = false,
         /// children inherit it