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-private.h

@@ -20,7 +20,7 @@
 #include <stdint.h>
 #include <time.h>
 
-#if defined(WIN32)
+#if defined(_WIN32)
 #include <basetsd.h>
 #define ssize_t SSIZE_T
 #endif
@@ -324,7 +324,7 @@ int iscsi_process_reject(struct iscsi_context *iscsi,
 				struct iscsi_in_pdu *in);
 int iscsi_send_target_nop_out(struct iscsi_context *iscsi, uint32_t ttt, uint32_t lun);
 
-#if defined(WIN32)
+#if defined(_WIN32)
 void iscsi_set_error(struct iscsi_context *iscsi, const char *error_string,
 		     ...);
 #else