Vlv2TbltDevicePkg/AcpiPlatform: Amend the declaration of mGlobalNvsArea

Vlv2TbltDevicePkg/AcpiPlatform: Amend the declaration of mGlobalNvsArea

mGlobalNvsArea wasn't declared as extern in AcpiPlatform.h and the header
was included in AcpiPlatform.c and AcpiPlatformHooks.c. Although it's
declared as extern in AcpiPlatformHooks.c, gcc still created an instance
of mGlobalNvsArea in AcpiPlatformHooks.c since the header was expanded
first and then the linker complained "multiple definition". This commit
rearrange the delaration of mGlobalNvsArea to satisfy gcc.

Cc: David Wei <david.wei@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: David Wei <david.wei@intel.com>