package/vlc: needs speexdsp for optional speex support

package/vlc: needs speexdsp for optional speex support

Fixes configure warning:

checking for SPEEX... yes
checking for SPEEXDSP... no
configure: WARNING: Package speexdsp was not found in the pkg-config search path.
Perhaps you should add the directory containing `speexdsp.pc'
to the PKG_CONFIG_PATH environment variable
Package 'speexdsp', required by 'world', not found.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>