platform/x86/intel: pmc/core: change pmc_lpm_modes to static

Sparse reports this issue
core.c: note: in included file:
core.h:239:12: warning: symbol 'pmc_lpm_modes' was not declared. Should it be static?

Global variables should not be defined in headers.  This only works
because core.h is only included by core.c. Single file use
variables should be static, so change its storage-class specifier
to static.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: David E. Box <david.e.box@linux.intel.com>
Link: https://lore.kernel.org/r/20220423123048.591405-1-trix@redhat.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>