libc/glibc: do not overwrite existing bits/syscall.h

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Rafael C <groups.r2@gmail.com>
Cc: Jérôme BARDON <bardon.pro@gmail.com>
Cc: Daniel Price <daniel.price@gmail.com>

reporter_name="Yann Diorcet <diorcet.yann@gmail.com>"

reporter_url=""

reporter_comment="Config to build cross-compiler with mingw as kernel type for

64 bits Windows target"