LibCore: Use generic AK_OS_BSD_GENERIC to hide Group::add_group()

This hides the method Group::add_group() on both MacOS and OpenBSD since
the function putgrent(), which is essential for add_group() to work, is
not available on these OSes.

Userland/Libraries/LibCore/Group.cpp

@@ -10,7 +10,7 @@
 
 namespace Core {
 
-#ifndef AK_OS_MACOS
+#ifndef AK_OS_BSD_GENERIC
 ErrorOr<void> Group::add_group(Group& group)
 {
     if (group.name().is_empty())

Userland/Libraries/LibCore/Group.h

@@ -15,7 +15,7 @@ namespace Core {
 
 class Group {
 public:
-#ifndef AK_OS_MACOS
+#ifndef AK_OS_BSD_GENERIC
     static ErrorOr<void> add_group(Group& group);
 #endif