loopfilter_16_msa: fix duplicate function name
loopfilter_16_msa: fix duplicate function name

with the rename in:
1dbe80bc7 Unify loopfilter function names

the first instance of aom_lpf_horizontal_16_dual_msa was shadowed.
rename this instance and make it private as was done in libvpx.

Change-Id: Ifadae6850c2b7c49ee8f4c988835464ecb3bde94

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom