Ignore -Wformat warning

Clang 6.0.1 update introduced a number of new warnings, failing
werror build. Ignore the -Wformat warning for external/dhcpcd-6.8.2.

Test: m checkbuild
Change-Id: Ia0624f0eedcf66c0c1ef6fa6479e895cf48d3662
diff --git a/Android.mk b/Android.mk
index 9d1d8c5..65a3d60 100644
--- a/Android.mk
+++ b/Android.mk
@@ -78,6 +78,8 @@
 LOCAL_CFLAGS += -Wno-unused-parameter
 # Compiler complains about possibly uninitialized variables in rpc-dbus.c.
 LOCAL_CFLAGS += -Wno-maybe-uninitialized
+# Compiler complains about incorrect format placeholders in dhcpcd.c.
+LOCAL_CFLAGS += -Wno-format
 LOCAL_SHARED_LIBRARIES += libc libcutils libnetutils