[gn build] Port ed27a4e

Author: llvmgnsyncbot

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -155,6 +155,7 @@ if (current_toolchain == default_toolchain) {
       "__algorithm/pstl_backends/cpu_backends/transform_reduce.h",
       "__algorithm/pstl_copy.h",
       "__algorithm/pstl_count.h",
+      "__algorithm/pstl_equal.h",
       "__algorithm/pstl_fill.h",
       "__algorithm/pstl_find.h",
       "__algorithm/pstl_for_each.h",