Fix a compiling error on armv6

The issue was caused by a bad merge in Change I5559d1e8

Change-Id: I6563f652bc1500202de361f8f51d11cc6ddf3331

vp8/encoder/dct.h

@@ -32,6 +32,15 @@ extern prototype_fdct(vp8_fdct_short4x4);
 #endif
 extern prototype_fdct(vp8_fdct_short8x4);
 
+// There is no fast4x4 (for now)
+#ifndef vp8_fdct_fast4x4
+#define vp8_fdct_fast4x4  vp8_short_fdct4x4_c
+#endif
+
+#ifndef vp8_fdct_fast8x4
+#define vp8_fdct_fast8x4  vp8_short_fdct8x4_c
+#endif
+
 #ifndef vp8_fdct_walsh_short4x4
 #define vp8_fdct_walsh_short4x4  vp8_short_walsh4x4_c
 #endif