dct/rdft: Add missing typedefs for context structs

Without the typedefs there can be trouble depending on #include order.

libavcodec/dct.h

@@ -28,7 +28,7 @@
 
 #include "rdft.h"
 
-struct DCTContext {
+typedef struct DCTContext {
     int nbits;
     int inverse;
     RDFTContext rdft;
@@ -36,7 +36,7 @@ struct DCTContext {
     FFTSample *csc2;
     void (*dct_calc)(struct DCTContext *s, FFTSample *data);
     void (*dct32)(FFTSample *out, const FFTSample *in);
-};
+} DCTContext;
 
 /**
  * Set up DCT.

libavcodec/rdft.h

@@ -48,7 +48,7 @@ extern SINTABLE(16384);
 extern SINTABLE(32768);
 extern SINTABLE(65536);
 
-struct RDFTContext {
+typedef struct RDFTContext {
     int nbits;
     int inverse;
     int sign_convention;
@@ -58,7 +58,7 @@ struct RDFTContext {
     SINTABLE_CONST FFTSample *tsin;
     FFTContext fft;
     void (*rdft_calc)(struct RDFTContext *s, FFTSample *z);
-};
+} RDFTContext;
 
 /**
  * Set up a real FFT.