selftests/resctrl: Fix compilation issues for other global variables

selftests/resctrl: Fix compilation issues for other global variables

Reinette reported following compilation issue on Fedora 32, gcc version
10.1.1

/usr/bin/ld: resctrl_tests.o:<src_dir>/resctrl.h:65: multiple definition
of `bm_pid'; cache.o:<src_dir>/resctrl.h:65: first defined here

Other variables are ppid, tests_run, llc_occup_path, is_amd. Compiler
isn't happy because these variables are defined globally in two .c files
but are not declared as extern.

To fix issues for the global variables, declare them as extern.

Chang Log:
- Split this patch from v4's patch 1 (Shuah).

Reported-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>