OvmfPkg/OvmfPkgX64: Expose DEBUG_TO_MEM as a build option

OvmfPkgX64.dsc has several build options, but some are
missing from the top definition list. This makes it difficult
for users to identify all available configuration flags.

Add DEBUG_TO_MEM to the [Defines] section with a default value of
FALSE to improve visibility.

This change has no functional impact.

Signed-off-by: Richard Lyu <richard.lyu@suse.com>

Author: r1chard-lyu

OvmfPkg/OvmfPkgX64.dsc

@@ -35,6 +35,7 @@
   DEFINE STANDALONE_MM_ENABLE    = FALSE
   DEFINE SOURCE_DEBUG_ENABLE     = FALSE
   DEFINE CC_MEASUREMENT_ENABLE   = TRUE
+  DEFINE DEBUG_TO_MEM            = FALSE
 
 !include OvmfPkg/Include/Dsc/OvmfTpmDefines.dsc.inc