Vlv2TbltDevicePkg: Only define MDEPKG_NDEBUG for gcc release build

Vlv2TbltDevicePkg: Only define MDEPKG_NDEBUG for gcc release build

We only enable "-Wno-unused-but-set-variable" for the release build
and gcc would complain that the varible passed to ASSERT_EFI_ERROR
wasn't used in the debug build. Just don't define MDEPKG_NDEBUG for
the debug build to make gcc happy with ASSERT_EFI_ERROR.

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Cc: David Wei <david.wei@intel.com>
CC: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>