__rdtscp() in Visual Studio takes one argument.

Fix a compilaton error under Visual Studio.

See https://docs.microsoft.com/en-us/cpp/intrinsics/rdtscp?view=vs-2015

Change-Id: I04c37aac3fea75e88db6ca2a51c2b4e570b83ed1
diff --git a/aom_ports/x86.h b/aom_ports/x86.h
index ebaf813..7c9790d 100644
--- a/aom_ports/x86.h
+++ b/aom_ports/x86.h
@@ -288,6 +288,9 @@
   unsigned int tscp;
   asm volatile("rdtscp\n\t" : "=a"(tscp) :);
   return tscp;
+#elif defined(_MSC_VER)
+  unsigned int ui;
+  return (unsigned int)__rdtscp(&ui);
 #else
 #if ARCH_X86_64
   return (unsigned int)__rdtscp();