iscsi: link libm into the module

Depending on the configuration of QEMU, some binaries might not need libm
at all.  In that case libiscsi, which uses exp(), will fail to load.
Link it in the module explicitly.

Reported-by: Yi Sun <yisun@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>