package/xen: fix build with gcc 11

Fix the following build failure with gcc 11:

linux.c:165:50: error: argument 7 of type 'const xen_pfn_t[]' {aka 'const long long unsigned int[]'} declared as an ordinary array [-Werror=vla-parameter]
  165 |                                  const xen_pfn_t arr[/*num*/], int err[/*num*/])
      |                                  ~~~~~~~~~~~~~~~~^~~~~~~~~~~~
In file included from linux.c:29:
private.h:35:50: note: previously declared as a variable length array 'const xen_pfn_t[num]' {aka 'const long long unsigned int[num]'}
   35 |                                  const xen_pfn_t arr[num], int err[num]);
      |                                  ~~~~~~~~~~~~~~~~^~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/f70d060cf50254694113f19f50a8ef96ef33dd1a



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>