speexdsp: 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: I8ea98e49b89e798cabb7d4055bbeceb08af90ff0

contrib/src/speexdsp/rules.mak

 # speexdsp
 
 SPEEXDSP_VERSION := git
-SPEEXDSP_HASH := HEAD
+SPEEXDSP_HASH := 83b63852e0e4755a8c04a1f9c87dd73b5aa6293f
 SPEEXDSP_GITURL := http://git.xiph.org/?p=speexdsp.git;a=snapshot;h=$(SPEEXDSP_HASH);sf=tgz
 
 PKGS += speexdsp