Add platform.cpp to Additional Build Files

(This is a merge of http://go/wvgerrit/70303)

The new platform.cpp file was not added to every build file that needs
to reference it. This was not detected as all of these platforms are
non-Windows, and platform.cpp is only required on Windows builds right
now. To avoid future problems should it ever provide non-Windows
functionality, though, I am adding it to the build files now.

Bug: 122953649
Test: Android Build
Change-Id: I57b983ad078dfef198b7c3e97fb93336b09704b6

libwvdrmengine/Android.mk

@@ -109,6 +109,7 @@ UTIL_SRC_DIR := cdm/util/src
 CORE_SRC_DIR := cdm/core/src
 LOCAL_SRC_FILES := \
     $(CORE_SRC_DIR)/properties.cpp \
+    $(UTIL_SRC_DIR)/platform.cpp \
     $(UTIL_SRC_DIR)/string_conversions.cpp \
     $(UTIL_SRC_DIR)/clock.cpp \
     $(UTIL_SRC_DIR)/file_store.cpp \