build: correct case for Windows

STREQUAL uses case sensitive comparisons, which is slightly faster for
the configuration phase.  Correct the case to ensure that the install
succeeds.

Author: compnerd

CMakeLists.txt

@@ -907,7 +907,7 @@ if(SWIFT_NEED_EXPLICIT_LIBDISPATCH)
     message(SEND_ERROR "libdispatch requires a newer clang compiler (${CMAKE_C_COMPILER_VERSION} < 3.9)")
   endif()
 
-  if(SWIFT_HOST_VARIANT_SDK STREQUAL Windows)
+  if(SWIFT_HOST_VARIANT_SDK STREQUAL WINDOWS)
     set(SOURCEKIT_LIBDISPATCH_RUNTIME_DIR bin)
   else()
     set(SOURCEKIT_LIBDISPATCH_RUNTIME_DIR lib)