Fix missing-prototype error in programs/fuzz by moving LLVMFuzzerTestOneInput prototype to common.h

Signed-off-by: Michael Schuster <michael@schuster.ms>

programs/fuzz/onefile.c

@@ -1,14 +1,13 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <stdio.h>
+#include "common.h"
 
 /* This file doesn't use any Mbed TLS function, but grab mbedtls_config.h anyway
  * in case it contains platform-specific #defines related to malloc or
  * stdio functions. */
 #include "mbedtls/build_info.h"
 
-int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
-
 int main(int argc, char **argv)
 {
     FILE *fp;