libmpeg2extractor: add dependency on libmedia_helper

libstagefright_mpeg2extractor references android::AudioParameter,
which is defined in libmedia_helper.  Builds with CFI drop the
dependency, likely due to implicitly enabling LTO.  Builds with musl
that don't support CFI fail with:

ld.lld: error: undefined symbol: vtable for android::AudioParameter
>>> referenced by AudioParameter.h:29 (frameworks/av/media/libmediahelper/include/media/AudioParameter.h:29)
>>>               Utils.o:(android::sendMetaDataToHal(android::sp<android::MediaPlayerBase::AudioSink>&, android::sp<android::MetaData> const&)) in archive out/soong/.intermediates/frameworks/av/media/libstagefright/libstagefright_mpeg2extractor/linux_musl_x86_64_static/libstagefright_mpeg2extractor.

Bug: 216192129
Test: m USE_HOST_MUSL=true host-native
Change-Id: I12e8580cc16eee643dbd890a920785680d3e4700