espeak: fix build on sparc64

sparc64 needs object files to be built with -fPIC in order to be
usable in shared libraries, otherwise the shared library cannot be
created.

Fixes:

  http://autobuild.buildroot.org/results/8b8/8b894f0bd42c18e7cda98c15480757f10d743423/



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