Mark win32's pthread_exit() as NORETURN

The pthread_exit() function is not expected to return. Ever. On Windows,
we call ExitThread() whose documentation claims: "Ends the calling
thread", i.e. there is no condition in which this function simply
returns: https://msdn.microsoft.com/en-us/library/windows/desktop/ms682659

While at it, fix the return type to be void, as per
http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_exit.html

Pointed out by Jeff King, helped by Stefan Naewe, Junio Hamano &
Johannes Sixt.

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

compat/win32/pthread.h

@@ -75,7 +75,7 @@ extern int win32_pthread_join(pthread_t *thread, void **value_ptr);
 #define pthread_equal(t1, t2) ((t1).tid == (t2).tid)
 extern pthread_t pthread_self(void);
 
-static inline int pthread_exit(void *ret)
+static inline void NORETURN pthread_exit(void *ret)
 {
 	ExitThread((DWORD)ret);
 }