libc/glibc: fix dubious construct when installing headers

This is dubious because if the copy fails, then we'll miss the error.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Thomas Petazzoni"

 reporter_url="http://www.free-electrons.com/"

 reporter_comment="Very basic testing only"