make libaudioutils_fixedfft vendor_available:true

make libaudioutils_fixedfft vendor_available:true

This, when used with BOARD_VNDK_VERSION flag, enables the
module to be built and installed twice, one for /system,
other for /vendor.

libaudioutils_fixedfft is used by libaudioutils which is in turn used by libs in
/vendor and thus marked as vendor_available. Therefore, libaudioutils_fixedfft
is also marked as vendor_available.

Details: https://android-review.googlesource.com/368372

Bug: 33241851
Test: BOARD_VNDK_VERSION=current m libaudioutils_fixedfft.vendor successful
Change-Id: Ic594eeb153b90ee45e7b5d9b682af9c3ff5471e4