build: Use glibc's xattr support instead of requiring libattr

Fixes the build on Debian, and is one library less.

Closes: #78

src/libostree/ostree-repo-checkout.c

@@ -23,7 +23,7 @@
 #include "config.h"
 
 #include <glib-unix.h>
-#include <attr/xattr.h>
+#include <sys/xattr.h>
 #include <gio/gfiledescriptorbased.h>
 #include <gio/gunixoutputstream.h>
 #include "otutil.h"

src/libostree/ostree-repo-commit.c

@@ -34,7 +34,7 @@
 #include "ostree-checksum-input-stream.h"
 #include "ostree-mutable-tree.h"
 #include "ostree-varint.h"
-#include <attr/xattr.h>
+#include <sys/xattr.h>
 #include <glib/gprintf.h>
 
 gboolean

src/libotutil/ot-fs-utils.c

@@ -22,7 +22,7 @@
 
 #include "ot-fs-utils.h"
 #include "libgsystem.h"
-#include <attr/xattr.h>
+#include <sys/xattr.h>
 #include <gio/gunixinputstream.h>
 
 int