Upgrade rust/crates/structopt to 0.3.18

Test: make
Change-Id: Ib5d515b0f5e9ac7fdd0f2f8e2f00ecf54dbf63ca
diff --git a/Android.bp b/Android.bp
index f836735..595a2f2 100644
--- a/Android.bp
+++ b/Android.bp
@@ -22,18 +22,17 @@
 //   ansi_term-0.11.0
 //   atty-0.2.14
 //   bitflags-1.2.1 "default"
-//   clap-2.33.1 "ansi_term,atty,color,default,strsim,suggestions,vec_map"
+//   clap-2.33.3 "ansi_term,atty,color,default,strsim,suggestions,vec_map"
 //   heck-0.3.1
 //   lazy_static-1.4.0
-//   libc-0.2.74
-//   proc-macro-error-1.0.3
-//   proc-macro-error-attr-1.0.3
-//   proc-macro2-1.0.19 "default,proc-macro"
+//   libc-0.2.77
+//   proc-macro-error-1.0.4 "default,syn,syn-error"
+//   proc-macro-error-attr-1.0.4
+//   proc-macro2-1.0.21 "default,proc-macro"
 //   quote-1.0.7 "default,proc-macro"
 //   strsim-0.8.0
-//   structopt-derive-0.4.7
-//   syn-1.0.36 "clone-impls,default,derive,full,parsing,printing,proc-macro,quote,visit"
-//   syn-mid-0.5.0
+//   structopt-derive-0.4.11
+//   syn-1.0.41 "clone-impls,default,derive,full,parsing,printing,proc-macro,quote,visit"
 //   textwrap-0.11.0
 //   unicode-segmentation-1.6.0
 //   unicode-width-0.1.8 "default"