Replace WIN32 with _WIN32

Using WIN32 depends on the build environment defining the variable.
_WIN32 is a predefined MSVC macro and is always available.

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

include/iscsi.h

@@ -20,7 +20,7 @@
 #include <stdint.h>
 #include <sys/types.h>
 
-#if defined(WIN32)
+#if defined(_WIN32)
 #define EXTERN __declspec( dllexport )
 #else
 #define EXTERN