doc/examples: fix lib math dep for decoding_encoding

It uses at least sin().
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/examples/Makefile

@@ -29,6 +29,7 @@ OBJS=$(addsuffix .o,$(EXAMPLES))
 
 # the following examples make explicit use of the math library
 avcodec:           LDLIBS += -lm
+decoding_encoding: LDLIBS += -lm
 muxing:            LDLIBS += -lm
 resampling_audio:  LDLIBS += -lm