meson: fix warning for bad sphinx-build

The warning was printing an empty string if the bad sphinx-build
was not passed on the command line.  Instead, always use the
path that was returned by find_program.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed