WIN32: fix calling conventions for 32-bit builds

On 32-bit Windows Intel builds the __cdecl and __stdcall calling
conventions are different so labeling the functions that are
exported or assigned to function pointers matters.

Change-Id: I03b6f34baeb9ffb2e683fd979f12f27a5078a4da

lib/kadm5/server_hooks.c

@@ -83,7 +83,7 @@ _kadm5_s_free_hooks(kadm5_server_context *ctx)
     _krb5_unload_plugins(ctx->context, "kadm5");
 }
 
-uintptr_t
+uintptr_t KRB5_LIB_CALL
 kadm5_get_instance(const char *libname)
 {
     static const char *instance = "libkadm5";