OSS-Fuzz: can't modify build commands easily, work around on our side

We modified the include path with recent changes, that isn't good for
OSS-Fuzz.  Lets make the path relative for including some semi-public
headers to allow OSS-Fuzz to build again.

Fix By: Brad House (@bradh352)

test/ares-test-fuzz.c

@@ -26,8 +26,8 @@
 #include <stddef.h>
 #include <stdio.h>
 #include "ares.h"
-#include "ares__buf.h"
+#include "include/ares__buf.h"
-#include "ares_mem.h"
+#include "include/ares_mem.h"
 
 int LLVMFuzzerTestOneInput(const unsigned char *data, unsigned long size);