mingw: squash another warning about a cast

MSys2's compiler is correct that casting a "void *" to a "DWORD" loses
precision, but in the case of pthread_exit() we know that the value
fits into a DWORD.

Just like casting handles to DWORDs, let's work around this issue by
casting to "intrptr_t" first, and immediately cast to the final type.

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

compat/win32/pthread.h

@@ -77,7 +77,7 @@ extern pthread_t pthread_self(void);
 
 static inline int pthread_exit(void *ret)
 {
-	ExitThread((DWORD)ret);
+	ExitThread((DWORD)(intptr_t)ret);
 }
 
 typedef DWORD pthread_key_t;