Fix for the Windows build

Change on 2015/01/09 by ctiller <ctiller@google.com>
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=83634893

include/grpc/support/port_platform.h

@@ -126,8 +126,8 @@
 #error Must define exactly one of GPR_ARCH_32, GPR_ARCH_64
 #endif
 
-#if defined(GPR_CPU_LINUX) + defined(GPR_CPU_POSIX) != 1
+#if defined(GPR_CPU_LINUX) + defined(GPR_CPU_POSIX) + defined(GPR_WIN32) != 1
-#error Must define exactly one of GPR_CPU_LINUX, GPR_CPU_POSIX
+#error Must define exactly one of GPR_CPU_LINUX, GPR_CPU_POSIX, GPR_WIN32
 #endif
 
 typedef int16_t gpr_int16;