Work around "error: variable 'uio' set but not used".

I've sent the real fix upstream, and will merge that when they commit it.

Change-Id: I75e5cab87026f8990c585d364dd178753d727e2a
diff --git a/libc/Android.mk b/libc/Android.mk
index 02fad3b..c0e1d32 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -711,6 +711,7 @@
     $(libc_common_cflags) \
     -Wno-sign-compare -Wno-uninitialized -Wno-unused-parameter \
     -Werror \
+    -Wno-error=unused-but-set-variable \
     -I$(LOCAL_PATH)/upstream-openbsd/android/include \
     -I$(LOCAL_PATH)/upstream-openbsd/lib/libc/include \
     -I$(LOCAL_PATH)/upstream-openbsd/lib/libc/gdtoa/ \