Adds explicit include to stdlib.h for abort

programs/fuzz/fuzz_privkey.c

@@ -1,4 +1,5 @@
 #include <stdint.h>
+#include <stdlib.h>
 #include "mbedtls/pk.h"
 
 //4 Kb should be enough for every bug ;-)

programs/fuzz/fuzz_pubkey.c

@@ -1,4 +1,5 @@
 #include <stdint.h>
+#include <stdlib.h>
 #include "mbedtls/pk.h"
 
 int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {