Replace include by forward declaration in test case

This should fix the remaining buildbot issue for the misleading identifier
detection pass.

Differential Revision: https://reviews.llvm.org/D112914

clang-tools-extra/test/clang-tidy/checkers/misc-misleading-identifier.cpp

@@ -1,6 +1,6 @@
 // RUN: %check_clang_tidy %s misc-misleading-identifier %t
 
-#include <stdio.h>
+int printf(const char *format, ...);
 
 // CHECK-MESSAGES: :[[#@LINE+1]]:1: warning: identifier has right-to-left codepoints [misc-misleading-identifier]
 short int א = (short int)0;