Rust: ignore .pdb files

MSVC builds of Rust on Windows automatically generate `*.pdb` files, which
store debugging information. For more information, see:

* https://github.com/rust-lang/rustlings/issues/275
* https://locka99.gitbooks.io/a-guide-to-porting-c-to-rust/content/debugging_rust/#microsoft-visual-studio

Rust.gitignore

@@ -9,3 +9,6 @@ Cargo.lock
 
 # These are backup files generated by rustfmt
 **/*.rs.bk
+
+# MSVC Windows builds of rustc generate these, which store debugging information
+*.pdb