Add `vendor_available: true` to libbuildverison

This allows using libbuildversion in Cuttlefish, which builds many
binaries for both vendor and host.

On vendor executables, this gets the build id from the
ro.vendor.build.version.incremental property.

Bug: 206843934
Test: m -j
Change-Id: I9ab11b42de3f1febf282f107c3e137592097d3c0
diff --git a/cc/libbuildversion/Android.bp b/cc/libbuildversion/Android.bp
index 2cff994..b105a30 100644
--- a/cc/libbuildversion/Android.bp
+++ b/cc/libbuildversion/Android.bp
@@ -19,4 +19,5 @@
         "//apex_available:platform",
         "//apex_available:anyapex",
     ],
+    vendor_available: true,
 }