doc: add `libsource-highlight-dev` to build prereqs
...as recent GDB uses it to highlight code when used in terminal.
--- a/doc/GDB.md Wed Jun 12 12:38:16 2019 +0100
+++ b/doc/GDB.md Wed Jun 12 20:20:18 2019 +0100
@@ -18,7 +18,7 @@
* On *Debian Buster* (tested on as of 2018-11-26)
```
- apt-get install build-essentials texinfo python3-dev flex bison libfl libexpat-dev
+ apt-get install build-essentials texinfo python3-dev flex bison libfl libexpat-dev libsource-highlight-dev
```
* On *Ubuntu 18.04* (tested on as of 2019-03-20)