vim-patch:5c9c5a4: runtime(c): syntax depends on the 'iskeyword' option

Problem:  C keywords are matched using the characters from 'iskeyword',
          so changing that option highlights part of an identifier as a
          keyword and leaves keywords containing an underscore
          unhighlighted.
Solution: Set the keyword characters with ":syn iskeyword".

fixes:  vim/vim#21173
closes: vim/vim#21176

https://github.com/vim/vim/commit/5c9c5a43c183e8d1e4156b468343d0aa0f3b081c

Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>
Co-Authored-By: Maxim Kim <habamax@gmail.com>

runtime/syntax/c.vim

@@ -1,7 +1,7 @@
 " Vim syntax file
 " Language:		C
 " Maintainer:		The Vim Project <https://github.com/vim/vim>
-" Last Change:		2026 Jul 25
+" Last Change:		2026 Aug 30
 " Former Maintainer:	Bram Moolenaar <Bram@vim.org>
 
 " Quit when a (custom) syntax file was already loaded
@@ -25,6 +25,13 @@ if exists("c_autodoc")
   unlet b:current_syntax
 endif
 
+" In VMS C keywords contain '$' characters.
+if has("vms")
+  syn iskeyword @,48-57,_,192-255,$
+else
+  syn iskeyword @,48-57,_,192-255
+endif
+
 " A bunch of useful C keywords
 syn keyword	cStatement	goto break return continue asm
 syn keyword	cLabel		case default