tools: hv: include string.h in hv_fcopy_daemon

The usage of strchr requires inclusion of string.h.

Fixes: 0c38cda6 ("tools: hv: remove unnecessary header files and netlink related code")
Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>