Makefile.unix: s/samplemuxer/sample_muxer/
Makefile.unix: s/samplemuxer/sample_muxer/

this matches the target name used in CMakeLists.txt and the writing app
stored in the output file

Change-Id: I161675c6bff8ebfa041331a329589441d3874514

kalli123/webm.libwebm,zofuthan/libwebm,Suvarna1488/webm.libwebm,webmproject/libwebm,matanbs/webm.libwebm,webmproject/libwebm,gshORTON/webm.libwebm,ericmckean/webm.libwebm,reimaginemedia/webm.libwebm,zofuthan/libwebm,kleopatra999/webm.libwebm,felipebetancur/libwebm,kim42083/webm.libwebm,iniwf/webm.libwebm,felipebetancur/libwebm,abwiz0086/webm.libwebm,webmproject/libwebm,Maria1099/webm.libwebm,Acidburn0zzz/webm.libwebm