Silence unused variable warning in sys_ptrace_test

This fixes the mips build broken by
I7d8a7d79585477d78da1f033c85f8d2cc3b34340.

Change-Id: Ib960bdb51ac81be068f1fdf702aa7c10b183f2a0
Test: lunch mips && cd bionic && mma
diff --git a/tests/sys_ptrace_test.cpp b/tests/sys_ptrace_test.cpp
index bee43ab..7483754 100644
--- a/tests/sys_ptrace_test.cpp
+++ b/tests/sys_ptrace_test.cpp
@@ -279,6 +279,7 @@
       << strerror(errno);
 #else
   (void)child;
+  (void)address;
 #endif
 }