When including the internal SLIRP library, we should add all the libraries that
it needs for the build.  Right now they are all included by QEMU, but -liphlpapi
is not needed without slirp.  Move it from LIBS to slirp_libs.

Based on a patch by Marc-André Lureau.

Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>