Modify `.gitignore` to ignore `Cargo.lock` only for `libtapasco`

.gitignore

@@ -54,9 +54,11 @@ webtalk.*
 # will have compiled files and executables
 /target/
 
-# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
-# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
+# Remove `Cargo.lock` from `.gitignore` if creating an executable, leave it for
+# libraries. More information here:
+# https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
+# So only ignore it for `libtapasco`:
+runtime/libtapasco/Cargo.lock
 
 # These are backup files generated by rustfmt
 **/*.rs.bk