winansi: use FLEX_ARRAY to avoid compiler warning

MSVC would complain thusly:

    C4200: nonstandard extension used: zero-sized array in struct/union

Let's just use the `FLEX_ARRAY` constant that we introduced for exactly
this type of scenario.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/winansi.c

@@ -546,7 +546,7 @@ static HANDLE swap_osfhnd(int fd, HANDLE new_handle)
 typedef struct _OBJECT_NAME_INFORMATION
 {
 	UNICODE_STRING Name;
-	WCHAR NameBuffer[0];
+	WCHAR NameBuffer[FLEX_ARRAY];
 } OBJECT_NAME_INFORMATION, *POBJECT_NAME_INFORMATION;
 
 #define ObjectNameInformation 1