patch 8.2.0320: no Haiku support

Problem:    No Haiku support.
Solution:   Add support for Haiku. (Emir Sari, closes #5605)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 508c59a..fb4db5d 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -10766,12 +10766,14 @@
 gui_gtk			Compiled with GTK+ GUI (any version).
 gui_gtk2		Compiled with GTK+ 2 GUI (gui_gtk is also defined).
 gui_gtk3		Compiled with GTK+ 3 GUI (gui_gtk is also defined).
+gui_haiku		Compiled with Haiku GUI.
 gui_mac			Compiled with Macintosh GUI.
 gui_motif		Compiled with Motif GUI.
 gui_photon		Compiled with Photon GUI.
 gui_running		Vim is running in the GUI, or it will start soon.
 gui_win32		Compiled with MS Windows Win32 GUI.
 gui_win32s		idem, and Win32s system being used (Windows 3.1)
+haiku			Haiku version of Vim.
 hangul_input		Compiled with Hangul input support. |hangul|
 hpux			HP-UX version of Vim.
 iconv			Can use iconv() for conversion.