speex: use fixed commit-id than HEAD tag

HEAD is a non-fixed commit-id. Builds based on it cannot be repoduced
as this Id is not trackable.
Replaced with latest commit-id availaible on master.

Tuleap: #762
Change-Id: I31bfdd6c99731acd3f0db3a41c69f1d9cd59f948

contrib/src/speex/rules.mak

 # speex
 
 SPEEX_VERSION := git
-SPEEX_HASH := HEAD
+SPEEX_HASH := 9172c7ef85fbf058027528d48ededbc7ca847908
 SPEEX_GITURL := http://git.xiph.org/?p=speex.git;a=snapshot;h=$(SPEEX_HASH);sf=tgz
 
 PKGS += speex