Silence a compiler warning due to unused variable on mips

oops, I did it again.

Test: lunch mips && cd bionic && mma
Change-Id: I1fe2cb847d0698b34766869f9626398dbcf81960
diff --git a/tests/sys_ptrace_test.cpp b/tests/sys_ptrace_test.cpp
index 8440adc..bce5898 100644
--- a/tests/sys_ptrace_test.cpp
+++ b/tests/sys_ptrace_test.cpp
@@ -242,6 +242,7 @@
   asm volatile("stp x0, x1, %0" : : "m"(data));
 #elif defined(__mips__)
 // TODO
+  UNUSED(data);
 #endif
 }