msvs: filter out include-only asm files

avoid building x86inc.asm, x86_abi_support.asm and vpx_config.asm as
they provide no symbols themselves

fixes:
 warning LNK4221: This object file does not define any previously
undefined public symbols, so it will not be used by any link operation
that consumes this library

Change-Id: Iecfe03aa76efbfc07c2af5b91ba5405634e45f1d