libc: add ftw / nftw functions

Please see "man 3 ftw" for a description of the
ftw / nftw functions.

This code is taken directly from netbsd unmodified.

Change-Id: Ia4879ac57212b424adf5281b5e92858e216d0f14
diff --git a/libc/Android.mk b/libc/Android.mk
index 2604404..6a77deb 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -331,6 +331,8 @@
 
 libc_upstream_netbsd_src_files := \
 	upstream-netbsd/libc/compat-43/creat.c \
+	upstream-netbsd/libc/gen/ftw.c \
+	upstream-netbsd/libc/gen/nftw.c \
 	upstream-netbsd/libc/gen/nice.c \
 	upstream-netbsd/libc/gen/psignal.c \
 	upstream-netbsd/libc/regex/regcomp.c \