Add x86_64 flag to guard fwd_txfm_ssse3.asm in make file

This fixes a VS build error. Fix by @johannkoenig.

Change-Id: I6e71435d70ae56079db7328e4c7915416ece8fda

vpx_dsp/vpx_dsp.mk

@@ -67,8 +67,10 @@ DSP_SRCS-yes            += fwd_txfm.c
 DSP_SRCS-yes            += fwd_txfm.h
 DSP_SRCS-$(HAVE_SSE2)   += x86/fwd_txfm_sse2.c
 DSP_SRCS-$(HAVE_SSE2)   += x86/fwd_txfm_impl_sse2.h
+ifeq ($(ARCH_X86_64),yes)
 ifeq ($(CONFIG_USE_X86INC),yes)
-DSP_SRCS-$(HAVE_SSSE3)  += x86/fwd_txfm_ssse3.asm
+DSP_SRCS-$(HAVE_SSSE3)  += x86/fwd_txfm_ssse3_x86_64.asm
+endif
 endif
 DSP_SRCS-$(HAVE_NEON)   += arm/fwd_txfm_neon.c
 endif  # CONFIG_VP9_ENCODER