Adding labeling for vendor security patch prop

This will allow adb shell getprop ro.vendor.build.security_patch to
properly return the correct build property, whereas previously it was
offlimits due to lack of label.

Test: adb shell getprop ro.vendor.build.security_patch successfully
returns whatever VENDOR_SECURITY_PATCH is defined to be in the Android
.mk files

Change-Id: Ie8427738125fc7f909ad8d51e4b76558f5544d49