Allow swscale emulation header to be included by C++ programs

Originally committed as revision 6027 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/swscale.h

 #ifndef SWSCALE_EMU_H
 #define SWSCALE_EMU_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* Dummy, only useful for compilation! */
 #define SWS_FAST_BILINEAR 1
 #define SWS_BILINEAR 2
@@ -29,4 +34,8 @@ int sws_scale(struct SwsContext *ctx, uint8_t* src[], int srcStride[],
 
 void sws_freeContext(struct SwsContext *swsContext);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* SWSCALE_EMU_H */