Honor MBEDTLS_CONFIG_FILE in fuzz tests

programs/fuzz/onefile.c

@@ -1,8 +1,15 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <stdio.h>
-// Get platform-specific definition
+
+/* This file doesn't use any Mbed TLS function, but grab config.h anyway
+ * in case it contains platform-specific #defines related to malloc or
+ * stdio functions. */
+#if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
 
 int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);