espeak : depends on MMU

Fixes http://autobuild.buildroot.net/results/dae/dae5c59dfa127e1c3f9e3885cdd1b97e54d8fe41/
and http://autobuild.buildroot.net/results/fa0/fa04a0fe7d7a201c5f733bd798c0da50ffb81271/


espeak uses fork(), don't work on non-MMU platforms

[Peter: indent with tab instead of spaces]
Signed-off-by: Arnaud Aujon <arnaud@intelibre.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>