std: make all dirent structs extern

Using structs with unspecified layout on the ABI boundry can't end well.

Author: ifreund

lib/std/c/emscripten.zig

@@ -171,7 +171,7 @@ pub const RTLD = struct {
     pub const LOCAL = 0;
 };
 
-pub const dirent = struct {
+pub const dirent = extern struct {
     ino: c_uint,
     off: c_uint,
     reclen: c_ushort,

lib/std/c/linux.zig

@@ -330,14 +330,14 @@ pub const RTLD = struct {
     pub const LOCAL = 0;
 };
 
-pub const dirent = struct {
+pub const dirent = extern struct {
     ino: c_uint,
     off: c_uint,
     reclen: c_ushort,
     type: u8,
     name: [256]u8,
 };
-pub const dirent64 = struct {
+pub const dirent64 = extern struct {
     ino: c_ulong,
     off: c_ulong,
     reclen: c_ushort,