libnamespaces: fix warning about syscall()

The warning was:
In file included from namespace.c:14:0:
../include/namespace.h: In function ‘setns’:
../include/namespace.h:37:2: warning: implicit declaration of function ‘syscall’ [-Wimplicit-function-declaration]

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
1 file changed