Fix regerror(..., nullptr, 0).

Found by passing a bad regular expression to the Google benchmark
code (https://github.com/google/benchmark).

Change-Id: I475db71c25706bbf02091b754acabe8254062f3a
diff --git a/libc/Android.mk b/libc/Android.mk
index 42f717e..f2004e1 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -850,7 +850,9 @@
 LOCAL_SRC_FILES := $(libc_upstream_netbsd_src_files)
 LOCAL_CFLAGS := \
     $(libc_common_cflags) \
-    -Wno-sign-compare -Wno-uninitialized \
+    -Wno-sign-compare \
+    -Wno-uninitialized \
+    -Wno-unused-parameter \
     -DPOSIX_MISTAKE \
     -include netbsd-compat.h \