Install libc6-dbg on linux slaves

Fixes 32-bit valgrind apparently

Author: alexcrichton

slaves/linux/Dockerfile

@@ -6,7 +6,7 @@ RUN apt-get install -y \
         curl make cmake xz-utils git \
         python-dev python-pip stunnel \
         g++-multilib libssl-dev libssl-dev:i386 gdb \
-        valgrind
+        valgrind libc6-dbg:i386
 
 # Install buildbot and prep it to run
 RUN pip install buildbot-slave