patch 9.1.0876: filetype: openCL files are not recognized

Problem:  filetype: openCL files are not recognized
Solution: detect '*.cl' files as opencl or lisp filetype,
          include a opencl syntax and filetype plugin (Wu, Zhenyu)

closes: #15825

Signed-off-by: Wu, Zhenyu <wuzhenyu@ustc.edu>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/autoload/dist/ft.vim

@@ -144,6 +144,14 @@ export def FTcfg()
   endif
 enddef
 
+export def FTcl()
+  if join(getline(1, 4), '') =~ '/\*'
+    setf opencl
+  else
+    setf lisp
+  endif
+enddef
+
 export def FTcls()
   if exists("g:filetype_cls")
     exe "setf " .. g:filetype_cls