glslang : Force update in update_external_sources

Linux's sh file didn't clobber previous external items synced.
This is having a problem with the previous changes which reverted
the glslang texel fetch patch, and then updated the revision.

Change-Id: I0e8e28d11fa40df00a5fb9164624f113b2adf084
diff --git a/update_external_sources.sh b/update_external_sources.sh
index 640a598..ea294f6 100755
--- a/update_external_sources.sh
+++ b/update_external_sources.sh
@@ -26,7 +26,7 @@
    echo "Updating $BASEDIR/glslang"
    cd $BASEDIR/glslang
    git fetch --all
-   git checkout $GLSLANG_REVISION
+   git checkout --force $GLSLANG_REVISION
 }
 
 function create_spirv-tools () {