freeswitch-stable: workaround libvpx compile issue

freeswitch-stable: workaround libvpx compile issue

The bundled libvpx got updated and now includes some extra tools. But
the linker is fed with some options it doesn't like, which results in
this:

/builder/shared-workdir/build/sdk/staging_dir/toolchain-x86_64_gcc-7.4.0_musl/bin/x86_64-openwrt-linux-musl-ld: unrecognized option '-Wl,-rpath-link=/builder/shared-workdir/build/sdk/staging_dir/target-x86_64_musl/usr/lib/libiconv-stub/lib'
/builder/shared-workdir/build/sdk/staging_dir/toolchain-x86_64_gcc-7.4.0_musl/bin/x86_64-openwrt-linux-musl-ld: use the --help option for usage information
/builder/shared-workdir/build/sdk/build_dir/target-x86_64_musl/freeswitch-1.10.0.-release/libs/libvpx/tools.mk:52: recipe for target 'tools/tiny_ssim' failed
make[6]: *** [tools/tiny_ssim] Error 1

freeswitch doesn't need these tools, it just needs the library, so this
commit just disables them.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>