package/asterisk: add optional dependency to speexdsp

Fixes
http://autobuild.buildroot.net/results/9f6/9f61192382ea4b57b0f0548626968752363bb13f/

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

package/asterisk/asterisk.mk

@@ -229,7 +229,7 @@ else
 ASTERISK_CONF_OPTS += --without-ssl
 endif
 
-ifeq ($(BR2_PACKAGE_SPEEX),y)
+ifeq ($(BR2_PACKAGE_SPEEX)$(BR2_PACKAGE_SPEEXDSP),yy)
 ASTERISK_DEPENDENCIES += speex
 ASTERISK_CONF_OPTS += --with-speex --with-speexdsp
 else