stat: use gnulib's alignof module

* src/stat.c (alignof): Remove definition.
Instead, include "alignof.h", and sort the #include directives.
And get its definition from the gnulib module by that name:
* bootstrap.conf (gnulib_modules): Add alignof.

bootstrap.conf

@@ -40,6 +40,7 @@ gnulib_modules="
   $avoided_gnulib_modules
   $obsolete_gnulib_modules
   acl
+  alignof
   alloca
   announce-gen
   areadlink-with-size

src/stat.c

@@ -57,18 +57,17 @@
 
 #include "system.h"
 
+#include "alignof.h"
+#include "areadlink.h"
 #include "error.h"
-#include "filemode.h"
 #include "file-type.h"
+#include "filemode.h"
 #include "fs.h"
 #include "getopt.h"
 #include "quote.h"
 #include "quotearg.h"
 #include "stat-time.h"
 #include "strftime.h"
-#include "areadlink.h"
-
-#define alignof(type) offsetof (struct { char c; type x; }, x)
 
 #if USE_STATVFS
 # define STRUCT_STATVFS struct statvfs