COMP: Avoid adding content_encoding.c twice.

Source/CTest/Curl/CMakeLists.txt

@@ -511,10 +511,6 @@ IF(NOT HAVE_ZLIB_H)
   SET(HAVE_LIBZ 0)
 ENDIF(NOT HAVE_ZLIB_H)
 
-IF(HAVE_LIBZ)
-  SET(libCurl_SRCS ${libCurl_SRCS} content_encoding.c)
-ENDIF(HAVE_LIBZ)
-
 IF(_FILE_OFFSET_BITS)
   SET(_FILE_OFFSET_BITS 64)
 ENDIF(_FILE_OFFSET_BITS)