.gitignore: add Rust related ignores

Cargo.lock: file that contains all (including transitive) the
dependencies. We only want to include one of it and it's yet available.

target*/: Cargo builds by default into the target/ dir, so avoid having
it in the git diff

!build.rs: Cargo has build scripts, that by default are named build.rs

.gitignore

@@ -20,6 +20,7 @@
 .dirstamp
 ABOUT-NLS
 aclocal.m4
+Cargo.lock
 ChangeLog
 compile
 config.status
@@ -50,6 +51,8 @@ patches/*
 
 # Ignore build dirs
 build*/
+target*/
 contrib-*
 install-*
+!build.rs
 !extras/package/apple/build.sh