Update OpenBSD build instructions following https://github.com/dolphin-emu/dolphin/pull/12716

Building-for-OpenBSD.md

@@ -1,7 +1,7 @@
 *OpenBSD is not officially supported. These instructions are not actively updated and represent what happened to work for one user on amd64 in March 2018.*
 
 *Games do not currently work on OpenBSD/amd64 due to [bug 10926][1]. Until/unless a better solution is devised, users can apply a patch like the following to get past the bug:*
-```
+```diff
 diff --git a/Source/Core/Common/MemArena.cpp b/Source/Core/Common/MemArena.cpp
 index 750a240e19..da77328e03 100644
 --- a/Source/Core/Common/MemArena.cpp
@@ -18,25 +18,14 @@ index 750a240e19..da77328e03 100644
 ```
 [1]: https://bugs.dolphin-emu.org/issues/10926
 
-Install the required packages: (Please amend if some are missing, I did not test this on a clean installation!)
-```
-pkg_add cmake pulseaudio qtbase
+Install the required packages. Keep in mind this is an incomplete list for a maximal build, the actual list of dependencies may be smaller depending on which features are enabled. Please amend as needed:
+```console
+# pkg_add bzip2 cmake curl enet gtest libhidapi libiconv libxkbcommon mbedtls miniupnpc ninja pulseaudio pugixml qtbase sdl2 sfml speexdsp zstd
 ```
 To build, run the following in the repository root directory:
+```console
+$ cmake -G Ninja -B build && cmake --build build -j$(getconf NPROCESSORS_ONLN)
 ```
-mkdir build && cd build
-export Qt5Gui_DIR=/usr/local/lib/qt5/cmake/Qt5Gui
-export Qt5Widgets_DIR=/usr/local/lib/qt5/cmake/Qt5Widgets
-cmake \
-    -DCMAKE_EXE_LINKER_FLAGS='-L/usr/local/lib -lintl -z wxneeded' \
-    -DENABLE_WX=OFF ..
-make
-```
-Rationale:
-+ Setting `Qt5Gui_DIR` and `Qt5Widgets_DIR` is necessary because CMake does not know to look for them under `/usr/local/lib/qt5/cmake` by default.
-+ PulseAudio wants to link with libintl, but it is not included in the default configuration.
-+ Linking with `-z wxneeded` is needed to enable writable-executable memory on OpenBSD. The resulting binary still needs to be run a partition with `wxallowed` set in the mount flags; `/usr/local` is mounted like this by default.
-+ wxWidgets needs to be disabled because wxWidgets3 has not been ported to OpenBSD yet. The experimental Qt interface is the only working GUI.
 
 Other considerations:
 + Dolphin needs a lot of memory to run - more than OpenBSD's default limits allow. Increase the `datasize` limits for your login class to at least 4G in `/etc/login.conf`, or (assuming your login class's `datasize-max` limit permits this) run `ulimit -d 4194304` before running Dolphin, e.g. in your `~/.profile` file or in a Dolphin startup script.