tests: fix xattr test build

Recent releases of attr don't ship attr/xattr.h anymore. It's included
in glibc >= 2.27 as sys/xattr.h. Check for sys/xattr.h and use that if
it exists.

configure.ac

@@ -44,6 +44,7 @@ AC_HEADER_STDC
 AC_CHECK_HEADERS( \
   getopt.h \
   pthread.h \
+  sys/xattr.h \
 )
 
 ##

tests/user/tsetxattr.c

@@ -16,7 +16,11 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <libgen.h>
-#include <attr/xattr.h>
+#if HAVE_SYS_XATTR_H
+    #include <sys/xattr.h>
+#else
+    #include <attr/xattr.h>
+#endif
 
 
 #include "9p.h"

tests/user/txattr.c

@@ -13,7 +13,11 @@
 #include <inttypes.h>
 #include <stdarg.h>
 #include <sys/types.h>
-#include <attr/xattr.h>
+#if HAVE_SYS_XATTR_H
+    #include <sys/xattr.h>
+#else
+    #include <attr/xattr.h>
+#endif
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <libgen.h>