libvpx

gen_msvs_vcxproj.sh: Set TargetName instead of OutputFile.

Set TargetName for library builds instead of changing the value of
OutputFile.

This fixes the following warnings:
warning MSB8012: TargetPath does not match Library's OutputFile property
value.

Change-Id: I4320b6d9ea922d3a15b9823c7c6694ee33edbf45