util/oslib-posix.c: Avoid warning on NetBSD

On NetBSD the compiler warns:
util/oslib-posix.c: In function 'sigaction_invoke':
util/oslib-posix.c:589:5: warning: missing braces around initializer [-Wmissing-braces]
     siginfo_t si = { 0 };
     ^
util/oslib-posix.c:589:5: warning: (near initialization for 'si.si_pad') [-Wmissing-braces]

because on this platform siginfo_t is defined as
  typedef union siginfo {
          char    si_pad[128];    /* Total size; for future expansion */
          struct _ksiginfo _info;
  } siginfo_t;

Avoid this warning by initializing the struct with {} instead;
this is a GCC extension but we use it all over the codebase already.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1500568341-8389-1-git-send-email-peter.maydell@linaro.org
