[gn build] Port 04d3a6b

Author: llvmgnsyncbot

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

@@ -496,6 +496,7 @@ if (current_toolchain == default_toolchain) {
       "__fwd/format.h",
       "__fwd/fstream.h",
       "__fwd/functional.h",
+      "__fwd/get.h",
       "__fwd/ios.h",
       "__fwd/istream.h",
       "__fwd/mdspan.h",
@@ -512,6 +513,7 @@ if (current_toolchain == default_toolchain) {
       "__fwd/string_view.h",
       "__fwd/subrange.h",
       "__fwd/tuple.h",
+      "__fwd/variant.h",
       "__fwd/vector.h",
       "__hash_table",
       "__ios/fpos.h",