libclang: add missing struct in the declaration

When building with compilers that do not support the Blocks extension,
we would fail to compile due to the missing type specifier on the
`typedef`.  This should repair those builds.

Fixes: llvm#62640
(cherry picked from commit d747f82)

Author: compnerd

clang/include/clang-c/Index.h

@@ -3705,7 +3705,7 @@ CINDEX_LINKAGE unsigned clang_visitChildren(CXCursor parent,
 typedef enum CXChildVisitResult (^CXCursorVisitorBlock)(CXCursor cursor,
                                                         CXCursor parent);
 #else
-typedef _CXChildVisitResult *CXCursorVisitorBlock;
+typedef struct _CXChildVisitResult *CXCursorVisitorBlock;
 #endif
 
 /**