zipl/libc: include 's390.h'

Include 'boot/s390.h' in 'libc.c' as `PAGE_SIZE` is used there and
move the 'libc.h' include directive to the top.

Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

zipl/boot/libc.c

@@ -9,12 +9,14 @@
  * it under the terms of the MIT license. See LICENSE for details.
  */
 
+#include "libc.h"
+
 #include <stdarg.h>
 
 #include "lib/zt_common.h"
+#include "boot/s390.h"
 
 #include "error.h"
-#include "libc.h"
 #include "sclp.h"
 
 extern char __heap_start[];