zycast: disable build on non-Linux OS

Fails with following errors on MacOS builder:

firmware-utils/src/zycast.c:205:35: error: use of undeclared identifier 'MSG_MORE'
                if (send(sockfd, phdr, HDRSIZE, MSG_MORE | MSG_DONTROUTE) < 0)
                                                ^
firmware-utils/src/zycast.c:285:39: error: use of undeclared identifier 'SO_BINDTODEVICE'
                        if (setsockopt(sockfd, SOL_SOCKET, SO_BINDTODEVICE,  optarg, strlen(optarg)) < 0)
                                                           ^

These seem to be Linux only definitions and there are no equivalents, so
leave rewrite of zycast to someone interested in running it on other OS.

Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>

Author: tmn505

CMakeLists.txt

@@ -110,7 +110,9 @@ FW_UTIL(uimage_sgehdr "" "" "${ZLIB_LIBRARIES}")
 FW_UTIL(wrt400n src/cyg_crc32.c "" "")
 FW_UTIL(xiaomifw "" "" "")
 FW_UTIL(xorimage "" "" "")
-FW_UTIL(zycast "" "" "")
+if (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+  FW_UTIL(zycast "" "" "")
+endif()
 FW_UTIL(zyimage "" "" "")
 FW_UTIL(zytrx "" "" "")
 FW_UTIL(zyxbcm "" "" "")