Don't build svdemo for Android and iOS.

f89296db5959cf009668f8aad85cfd860b9252ee adds building of svdemo,
this however uses SAL_IMPLEMENT_MAIN from include/sal/main.h --
which specifically can't be built on Android and iOS, hence
we shouldn't build svdemo for those platforms.

Change-Id: I6ce851ece90837112279c89467326b82f7a2070b
Reviewed-on: https://gerrit.libreoffice.org/9391
Tested-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>