lazyload.h: fix warnings about mismatching function pointer types

Here, GCC warns about every use of the INIT_PROC_ADDR macro, for example:

In file included from compat/mingw.c:8:
compat/mingw.c: In function 'mingw_strftime':
compat/win32/lazyload.h:38:12: warning: assignment to
   'size_t (*)(char *, size_t,  const char *, const struct tm *)'
   {aka 'long long unsigned int (*)(char *, long long unsigned int,
      const char *, const struct tm *)'} from incompatible pointer type
   'FARPROC' {aka 'long long int (*)()'} [-Wincompatible-pointer-types]
   38 |  (function = get_proc_addr(&proc_addr_##function))
      |            ^
compat/mingw.c:1014:6: note: in expansion of macro 'INIT_PROC_ADDR'
 1014 |  if (INIT_PROC_ADDR(strftime))
      |      ^~~~~~~~~~~~~~

(message wrapped for convenience). Insert a cast to keep the compiler
happy. A cast is fine in these cases because they are generic function
pointer values that have been looked up in a DLL.

Helped-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/win32/lazyload.h

@@ -26,7 +26,8 @@ struct proc_addr {
 #define DECLARE_PROC_ADDR(dll, rettype, function, ...) \
 	static struct proc_addr proc_addr_##function = \
 	{ #dll, #function, NULL, 0 }; \
-	static rettype (WINAPI *function)(__VA_ARGS__)
+	typedef rettype (WINAPI *proc_type_##function)(__VA_ARGS__); \
+	static proc_type_##function function
 
 /*
  * Loads a function from a DLL (once-only).
@@ -35,7 +36,7 @@ struct proc_addr {
  * This function is not thread-safe.
  */
 #define INIT_PROC_ADDR(function) \
-	(function = get_proc_addr(&proc_addr_##function))
+	(function = (proc_type_##function)get_proc_addr(&proc_addr_##function))
 
 static inline FARPROC get_proc_addr(struct proc_addr *proc)
 {