tools/ocaml: Fix build error with CentOS 7

tools/ocaml: Fix build error with CentOS 7

gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-28) complains:

  xenctrl_stubs.c: In function 'stub_xc_domain_create':
  xenctrl_stubs.c:216:28: error: 'val' may be used uninitialized
                          in this function [-Werror=maybe-uninitialized]
     cfg.arch.emulation_flags = ocaml_list_to_c_bitmap
                              ^
  xenctrl_stubs.c:198:12: error: 'val' may be used uninitialized
                          in this function [-Werror=maybe-uninitialized]
    cfg.flags = ocaml_list_to_c_bitmap
              ^
  cc1: all warnings being treated as errors

GCC doesn't point at the correct piece of code, but the diagnostic text is
correct, and can occur when the list is empty. Initialise val to 0.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>