net/slirp.c: fix warning with _FORTIFY_SOURCE

net/slirp.c: fix warning with _FORTIFY_SOURCE

CC    net/slirp.o
cc1: warnings being treated as errors
net/slirp.c: In function 'slirp_smb_cleanup':
net/slirp.c:470: error: ignoring return value of 'system', declared with attribute warn_unused_result
make: *** [net/slirp.o] Error 1

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>