fix(ff-a): missing plat_ffa module at toolchain "android_aarch64"

Change-Id: I461f29f5199c45b3f243d2e83f2a0e318c22f909
Signed-off-by: J-Alves <joao.alves@arm.com>
diff --git a/BUILD.gn b/BUILD.gn
index d6d9fe9..6027292 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -233,6 +233,7 @@
   max_vms = 16
   toolchain_args = {
     pl011_base_address = "0x09000000"
+    plat_ffa = "//src/arch/aarch64/plat/ffa:hypervisor"
   }
 }