shared: ensure "nm-glib.h" is not used without "nm-macros-internal.h"

Note how "nm-glib.h" uses _nm_printf() macro, which is defined in
"nm-macros-internal.h". There are many ways how to solve that
problem.

For example, we could define certain _nm_*() macros in "nm-glib.h"
itself. However, that is a bit awkward, because optimally "nm-glib.h"
only provides functions that are strictly related to glib compatiblity.
_nm_printf() is used by "nm-glib.h" for its own implementation, it should
not provide (or reimplement) this macro.

We solve this instead by enforcing what NetworkManager already does.
NetworkManager never includes "nm-glib.h" directly, instead
"nm-macros-internal.h" is the only place that includes the glib compat
header. It means, you cannot use "nm-glib.h" without
"nm-macros-internal.h". But that is a reasonable compromise, with
respect to granularity. The granularity at the moment is: if you use
anything from "shared/nm-utils", you almost always need at least
"nm-macros-internal.h" header (and automatically also get "nm-glib.h"
and "gsystem-local-alloc.h"). It's not intended, to use "nm-glib.h"
directly.

This makes "nm-glib.h" an implementation detail of "nm-macros-internal.h"
and it only exists to separate code that is related to glib compatibility to
its own header.

shared/nm-utils/nm-glib.h

@@ -20,6 +20,14 @@
 #ifndef __NM_GLIB_H__
 #define __NM_GLIB_H__
 
+/*****************************************************************************/
+
+#ifndef __NM_MACROS_INTERNAL_H__
+#error "nm-glib.h requires nm-macros-internal.h. Do not include this directly"
+#endif
+
+/*****************************************************************************/
+
 #include <gio/gio.h>
 #include <string.h>