ANDROID: modpost: Check for KBUILD_MIXED_TREE for Modules.symvers

When building with KBUILD_MIXED_TREE, Modules.symvers will not be built
since vmlinux.symvers won't be in the normal build outputs. Fix this by
checking for the mixed-build-prefix in the Modules.symvers target.

Fixes: d0736af8 ("kbuild: generate Module.symvers only when vmlinux exists")
Signed-off-by: J. Avila <elavila@google.com>
Change-Id: Ic8dcc220cb7b93498629719aaccdb7b6fc38a6a1