libtevent: add missing dependency on libcap

The built-in library libreplace requires it,
or it fails with:
../lib/replace/system/capability.h:40:28: fatal error: sys/capability.h: No such file or directory

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>