Rename headers to end with _p.h

Frameworks have a convention of naming uninstalled headers in src/ with
a _p at the end of the name, to make it clear they are not part of the
API. None of the headers in KImageFormats are installed, so it is not
really necessary to follow this convention, but we follow it anyway for
the benefit of both humans and tools (like kapidox).

src/imageformats/hdr_p.h

@@ -0,0 +1,21 @@
+/* This file is part of the KDE project
+   Copyright (C) 2005 Christoph Hormann <chris_hormann@gmx.de>
+
+   This program is free software; you can redistribute it and/or
+   modify it under the terms of the Lesser GNU General Public
+   License as published by the Free Software Foundation; either
+   version 2 of the License, or (at your option) any later version.
+*/
+
+#ifndef KIMG_HDR_H
+#define KIMG_HDR_H
+
+class QImageIO;
+
+extern "C" {
+    void kimgio_hdr_read(QImageIO *);
+    void kimgio_hdr_write(QImageIO *);
+}
+
+#endif
+