speex: make it compilable in Thumb-2 mode

This fixes build failures such as
http://autobuild.buildroot.org/results/901e8fe6df0e568624f1ceffc1979c5010e19328/build-end.log

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>