doc: Add bash as an OpenBSD dependency

If we require Python for the test framework, we should also require
bash. It is required for the linters and other scripts and does not
comes in a default OpenBSD installation.

doc/build-openbsd.md

@@ -15,6 +15,7 @@ pkg_add qt5 # (optional for enabling the GUI)
 pkg_add autoconf # (select highest version, e.g. 2.69)
 pkg_add automake # (select highest version, e.g. 1.16)
 pkg_add python # (select highest version, e.g. 3.8)
+pkg_add bash
 
 git clone https://github.com/bitcoin/bitcoin.git
 ```