Use correct tag names for compatability with fwd decls.
Some old code checks that _WINDEF_ is defined in windef.h.

diff --git a/include/winbase.h b/include/winbase.h
index 1dc02bc..fb96b34 100644
--- a/include/winbase.h
+++ b/include/winbase.h
@@ -673,7 +673,7 @@
 } SYSTEM_POWER_STATUS, *LPSYSTEM_POWER_STATUS;
 
 
-typedef struct tagSYSTEM_INFO
+typedef struct _SYSTEM_INFO
 {
     union {
 	DWORD	dwOemId; /* Obsolete field - do not use */