enc_fuzzer.cc: remove duplicate <cstdlib> include

after:
98c2780 IWYU: Include all headers for symbols used in files.

clears a clang-tidy warning

Change-Id: Ib9190305bc059c69c7f1f7abf52760eb308bfa35

Author: jzern

tests/fuzzer/enc_fuzzer.cc

@@ -17,7 +17,6 @@
 #include <cstddef>
 #include <cstdint>
 #include <cstdlib>
-#include <cstdlib>
 #include <iostream>
 #include <string>
 #include <string_view>