Fix IOS builds (#127985)

IOS builds fail these days, fix it.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/127985
Approved by: https://github.com/ezyang
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b36f52c..cd11ffd 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -898,7 +898,7 @@
 endif()
 
 # Enable sleef on macOS with Apple silicon by default
-if((${CMAKE_SYSTEM_NAME} STREQUAL "Darwin") AND (${CMAKE_SYSTEM_PROCESSOR} STREQUAL "arm64"))
+if((${CMAKE_SYSTEM_NAME} STREQUAL "Darwin") AND ("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "arm64"))
   message(STATUS "Running on macOS with Apple silicon")
   string(APPEND CMAKE_CXX_FLAGS " -DAT_BUILD_ARM_VEC256_WITH_SLEEF")
   add_definitions(-DAT_BUILD_ARM_VEC256_WITH_SLEEF)