[Polly][test] Add dependency to count.

Polly does not use the count program itself, but somewhere in lit it is
expected to exists. Otherwise, the following error occurs:

    llvm-lit: llvm-project/llvm/utils/lit/lit/llvm/subst.py:133: fatal: Did not find count in ./bin

polly/test/CMakeLists.txt

@@ -34,7 +34,7 @@ else ()
   set(LLVM_FILECHECK "${LLVM_TOOLS_BINARY_DIR}/FileCheck")
   set(LLVM_NOT "${LLVM_TOOLS_BINARY_DIR}/not")
   set(POLLY_TEST_EXTRA_PATHS "")
-  set(POLLY_TEST_DEPS llvm-config opt LLVMPolly FileCheck not)
+  set(POLLY_TEST_DEPS llvm-config opt LLVMPolly FileCheck not count)
 endif()
 
 if (POLLY_BUNDLED_ISL)