Make libqcomvoiceprocessing build with the VNDK.

When the vndk is enabled (BOARD_VNDK_VERSION=current),
vendor libraries must only link against vendor variants
in the build system. This means that they receive restricted
sets of headers and that automatic global headers are restricted.

Test: build libqcomvoiceprocessing with BOARD_VNDK_VERSION=current
Bug: 37342627
Change-Id: I1500835ddf92bcff624281dea88e6da678064646