tests/docker: Install static libc package in CentOS 7

tests/docker: Install static libc package in CentOS 7

We need to install the static libc package to be able to run
the TCG tests:

  $ make check-tcg
  ...
    BUILD   TCG tests for x86_64-softmmu
    BUILD   x86_64-softmmu guest-tests with cc
  /usr/bin/ld: hello: warning: allocated section `.notes' not in segment
  /usr/bin/ld: memory: warning: allocated section `.notes' not in segment
    BUILD   TCG tests for x86_64-linux-user
    BUILD   x86_64-linux-user guest-tests with cc
  /usr/bin/ld: cannot find -lpthread
  /usr/bin/ld: cannot find -lc
  collect2: error: ld returned 1 exit status
  make[2]: *** [threadcount] Error 1
  make[1]: *** [cross-build-guest-tests] Error 2
  make: *** [build-tcg-tests-x86_64-linux-user] Error 2

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210121172829.1643620-2-f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>