package/pulseaudio: fix speex dependency

pulseaudio needs speexdsp, not speex resulting in the following build
failure since commit 7752afbe which
explicitly enable or disable speex:

../output-1/build/pulseaudio-16.1/meson.build:713:2: ERROR: Dependency "speexdsp" not found, tried pkgconfig and cmake

Fixes:
 - http://autobuild.buildroot.org/results/8d4c5d5bfd65744753ca75180dd45a01c90f0b91



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>