Namespace the idct/iad symbols

Make linking to libvpx and libaom at the same time possible.

Change-Id: I7bab8527a32e446e3d564e6fa5d94ccd056bc63f
diff --git a/aom_dsp/inv_txfm.h b/aom_dsp/inv_txfm.h
index 0f84e38..c3d794e 100644
--- a/aom_dsp/inv_txfm.h
+++ b/aom_dsp/inv_txfm.h
@@ -97,13 +97,13 @@
 #endif  // CONFIG_AOM_HIGHBITDEPTH
 #endif  // CONFIG_EMULATE_HARDWARE
 
-void idct4_c(const tran_low_t *input, tran_low_t *output);
-void idct8_c(const tran_low_t *input, tran_low_t *output);
-void idct16_c(const tran_low_t *input, tran_low_t *output);
-void idct32_c(const tran_low_t *input, tran_low_t *output);
-void iadst4_c(const tran_low_t *input, tran_low_t *output);
-void iadst8_c(const tran_low_t *input, tran_low_t *output);
-void iadst16_c(const tran_low_t *input, tran_low_t *output);
+void aom_idct4_c(const tran_low_t *input, tran_low_t *output);
+void aom_idct8_c(const tran_low_t *input, tran_low_t *output);
+void aom_idct16_c(const tran_low_t *input, tran_low_t *output);
+void aom_idct32_c(const tran_low_t *input, tran_low_t *output);
+void aom_iadst4_c(const tran_low_t *input, tran_low_t *output);
+void aom_iadst8_c(const tran_low_t *input, tran_low_t *output);
+void aom_iadst16_c(const tran_low_t *input, tran_low_t *output);
 
 #if CONFIG_AOM_HIGHBITDEPTH
 void aom_highbd_idct4_c(const tran_low_t *input, tran_low_t *output, int bd);