travis.yml: Add missing clang-10 package to the 'Clang (disable-tcg)' job

travis.yml: Add missing clang-10 package to the 'Clang (disable-tcg)' job

Since commit 74a1b256d775("configure: Bump minimum Clang version to 10.0") qemu
needs Clang version 10.0 as the minimum version to build qemu with
Clang. However 'focal' ships by default with Clang version 7.0.0 which causes an
error while executing the 'Clang (disable-tcg)' travis job of the form below:

<snip>
$clang --version

clang version 7.0.0 (tags/RELEASE_700/final)

<snip>
 ERROR: You need at least GCC v7.4 or Clang v10.0 (or XCode Clang v12.0)

 # QEMU configure log Fri 14 Apr 2023 03:48:22 PM UTC

 # Configured with: '../configure' '--disable-docs' '--disable-tools'
 '--disable-containers' '--disable-tcg' '--enable-kvm' '--disable-tools'
 '--enable-fdt=system' '--host-cc=clang' '--cxx=clang++'

Fix this by adding 'clang-10' to the 'apt_packages' section of the "[s390x]
Clang (disable-tcg)" job and updating the compiler to 'clang-10'.

Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Message-Id: <20230414210645.820204-1-vaibhav@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>