espeak: disable for static-only builds

The Makefile wants to unconditionally build a shared library, and
since it's probably not a package that relevant in static-only
scenarios, let's simply disable it.

Fixes:

  http://autobuild.buildroot.org/results/b84/b84f56cdc22f333b95e0894084a800be53aefd26/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>