runtime(c): Recognize "__inline" (#14145)

`__inline` is recognized by GCC, and may even be preferred, as MSVC does
not recognize `__inline__`.

Signed-off-by: Wu Yongwei <wuyongwei@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/c.vim b/runtime/syntax/c.vim
index a593bd2..30db943 100644
--- a/runtime/syntax/c.vim
+++ b/runtime/syntax/c.vim
@@ -252,7 +252,7 @@
   syn keyword	cOperator	typeof __typeof__
   syn keyword	cOperator	__real__ __imag__
   syn keyword	cStorageClass	__attribute__ __const__ __extension__
-  syn keyword	cStorageClass	inline __inline__
+  syn keyword	cStorageClass	inline __inline __inline__
   syn keyword	cStorageClass	__restrict__ __volatile__ __noreturn__
 endif
 syn keyword	cType		int long short char void