glibc: use oe_multilib_header on bits/floatn.h

glibc: use oe_multilib_header on bits/floatn.h

When building SDK via populate_sdk for qemuarm64 with multilib
enabled, we would have conflict about bits/floatn.h at populate_sdk
time.

  file /usr/include/bits/floatn.h conflicts between attempted ins
talls of libc6-dev-2.27-r0.aarch64 and lib32-libc6-dev-2.27-r0.armv7vehf_vfp

Apply oe_multilib_header on this header file to fix the problem.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>