meson: Declare have_virtfs_proxy_helper in main meson.build

have_virtfs_proxy_helper is used from docs/meson.build, and can be
not declared when including it before fsdev/meson.build. This fixes:

  ../docs/meson.build:54:2: ERROR: Unknown variable "have_virtfs_proxy_helper".

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210120151539.1166252-1-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>