Allow SLOG* to vendors

Not only vendor libs, but also Android libs that are marked as
vendor_available are using SLOG*. Since both pure vendor libs and
Android libs available to vendors are using the same liblog headers for
VNDK, SLOG* needs to be visible via the headers.

Bug: 63088165
Test: BOARD_VNDK_VERSION=current m -j ATFWD-daemon
(or any other vendor modules using SLOG*)

Change-Id: Idfb74887a42d2418d3896bfdf5be3b4fc8e4c27f