doc: Suggest only necessary Qt packages for installation on OpenBSD

The currently suggested `qt5` installs many unneeded dependencies, for
example, `qtsensors`, `qtspeech` etc.

Author: hebasto

doc/build-openbsd.md

@@ -1,6 +1,6 @@
 # OpenBSD Build Guide
 
-**Updated for OpenBSD [7.4](https://www.openbsd.org/74.html)**
+**Updated for OpenBSD [7.5](https://www.openbsd.org/75.html)**
 
 This guide describes how to build bitcoind, command-line utilities, and GUI on OpenBSD.
 
@@ -63,7 +63,7 @@ export BDB_PREFIX="/path/to/bitcoin/depends/x86_64-unknown-openbsd"
 Bitcoin Core includes a GUI built with the cross-platform Qt Framework. To compile the GUI, Qt 5 is required.
 
 ```bash
-pkg_add qt5
+pkg_add qtbase qttools
 ```
 
 ## Building Bitcoin Core