Guard DECLS(ssse3), FNS(ssse3) with #if HAVE_SSSE3

The aom_dsp/x86/variance_sse2.c file is compiled if the ENABLE_SSE2
cmake option is enabled. The file also contains some SSSE3 code. That
code should only be compiled if the ENABLE_SSSE3 cmake option is
enabled.

Bug: aomedia:3578
Change-Id: I86685862e7da9506bd551b76228cdc9920c68ab1

aom_dsp/x86/variance_sse2.c

diff --git a/aom_dsp/x86/variance_sse2.c b/aom_dsp/x86/variance_sse2.c
index 81b3007..e71244f 100644
--- a/aom_dsp/x86/variance_sse2.c
+++ b/aom_dsp/x86/variance_sse2.c
@@ -415,7 +415,9 @@
   DECL(8, opt);    \
   DECL(16, opt)
 
+#if HAVE_SSSE3
 DECLS(ssse3);
+#endif
 #undef DECLS
 #undef DECL
 
@@ -491,7 +493,9 @@
   FN(4, 4, 4, 2, 2, opt, (int32_t), (int32_t))
 #endif
 
+#if HAVE_SSSE3
 FNS(ssse3)
+#endif
 
 #undef FNS
 #undef FN
@@ -508,7 +512,9 @@
   DECL(8, opt);    \
   DECL(16, opt)
 
+#if HAVE_SSSE3
 DECLS(ssse3);
+#endif
 #undef DECL
 #undef DECLS
 
@@ -588,7 +594,9 @@
   FN(4, 4, 4, 2, 2, opt, (uint32_t), (int32_t))
 #endif
 
+#if HAVE_SSSE3
 FNS(ssse3)
+#endif
 
 #undef FNS
 #undef FN