make: switch from -I to -iquote

Our rule right now is to use <> for external headers,
"" for internal ones. The idea was to avoid conflicts
between e.g. a system file named <trace.h> and an
internal one by the same name.

Unfortunately we use -I compiler flag so it does not
help: a system file doing #include <trace.h> will
still pick up ours first.

To fix, switch to -iquote which is supported by both
gcc and clang and only affects #include "" directives.

As a side effect, this catches any future uses of
 #include <> for internal headers.

Suggested-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>