build: correct case for STREQUAL (NFC)

The swift variables use the upper case spelling while CMake uses mixed
case.  Use the correct case to fix the build.  This is preferable to
using MATCH to avoid the unnecessary configure time penalties.

Author: compnerd

stdlib/public/core/CMakeLists.txt

@@ -284,7 +284,7 @@ if(SWIFT_BUILD_STATIC_STDLIB AND "${SWIFT_HOST_VARIANT_SDK}" STREQUAL "LINUX")
    list(APPEND swift_core_private_link_libraries swiftImageInspectionShared)
 endif()
 
-if(SWIFT_PRIMARY_VARIANT_SDK STREQUAL Windows)
+if(SWIFT_PRIMARY_VARIANT_SDK STREQUAL WINDOWS)
   list(APPEND swift_core_private_link_libraries shell32)
 endif()