[Minor] remove check for sys/cdefs.h in CMakelists.txt

rspamd uses G_(BEGIN|END)_DECLS not __(BEGIN|END)_DECLS

CMakeLists.txt

@@ -284,7 +284,6 @@ CHECK_INCLUDE_FILES(sys/un.h  HAVE_SYS_UN_H)
 CHECK_INCLUDE_FILES(sys/stat.h  HAVE_SYS_STAT_H)
 CHECK_INCLUDE_FILES(sys/wait.h  HAVE_SYS_WAIT_H)
 CHECK_INCLUDE_FILES(sys/param.h  HAVE_SYS_PARAM_H)
-CHECK_INCLUDE_FILES(sys/cdefs.h  HAVE_SYS_CDEFS_H)
 CHECK_INCLUDE_FILES(sys/file.h  HAVE_SYS_FILE_H)
 CHECK_INCLUDE_FILES(sys/resource.h  HAVE_SYS_RESOURCE_H)
 CHECK_INCLUDE_FILES(netinet/in.h  HAVE_NETINET_IN_H)