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

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

The member "u.L" is an array, and the current initializer triggers

  StdLib/LibC/gdtoa/strtof.c: In function '_strtof':
  StdLib/LibC/gdtoa/strtof.c:53:9: error: missing braces around
                                   initializer [-Werror=missing-braces]
  StdLib/LibC/gdtoa/strtof.c:53:9: error: (near initialization for
                                   'u.L') [-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@15785 6f19259b-4bc3-4df7-8a09-765794883524