Add libavcodec to compiler include flags in order to simplify header

include paths in the source files.
mostly from a patch by Ronald S. Bultje, rbultje ronald.bitfreak net

Originally committed as revision 9034 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/ps2/dsputil_mmi.c

@@ -22,7 +22,7 @@
  * clear_blocks_mmi() by BroadQ
  */
 
-#include "../dsputil.h"
+#include "dsputil.h"
 #include "mmi.h"
 
 void ff_mmi_idct_put(uint8_t *dest, int line_size, DCTELEM *block);

libavcodec/ps2/idct_mmi.c

@@ -26,8 +26,8 @@
  *
 */
 
-#include "../common.h"
-#include "../dsputil.h"
+#include "common.h"
+#include "dsputil.h"
 #include "mmi.h"
 
 #define BITS_INV_ACC    5       // 4 or 5 for IEEE

libavcodec/ps2/mpegvideo_mmi.c

@@ -20,9 +20,9 @@
  * MMI optimization by Leon van Stuivenberg
  */
 
-#include "../dsputil.h"
-#include "../mpegvideo.h"
-#include "../avcodec.h"
+#include "dsputil.h"
+#include "mpegvideo.h"
+#include "avcodec.h"
 
 static void dct_unquantize_h263_mmi(MpegEncContext *s,
                                   DCTELEM *block, int n, int qscale)