StdLib/LibC/Stdio: fix "missing braces around initializer"

The member "fext._ub" is a structure (of type "struct __sbuf"), and the
current initializer triggers

  StdLib/LibC/Stdio/vswscanf.c: In function 'vswscanf':
  StdLib/LibC/Stdio/vswscanf.c:75:10: error: missing braces around
                                      initializer [-Werror=missing-braces]
  StdLib/LibC/Stdio/vswscanf.c:75:10: error: (near initialization for
                                      'fext._ub') [-Werror=missing-braces]
  cc1: all warnings being treated as errors

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15786 6f19259b-4bc3-4df7-8a09-765794883524