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/tga.cpp

@@ -18,7 +18,7 @@
  *     uncompressed true color tga files
  */
 
-#include "tga.h"
+#include "tga_p.h"
 
 #include <assert.h>