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

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

(cherry picked from commit cac2908b08f517802e719ddafe39f45c85c96a33)

Change-Id: I317a7c2ea6535998c0853029023fcefc88cb3750
diff --git a/libc/Android.mk b/libc/Android.mk
index d94dce2..1ca84c1 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -856,7 +856,9 @@
 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 \