[gn build] Port f9d6d6f

Author: llvmgnsyncbot

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

@@ -505,7 +505,6 @@ if (current_toolchain == default_toolchain) {
       "__fwd/array.h",
       "__fwd/bit_reference.h",
       "__fwd/fstream.h",
-      "__fwd/get.h",
       "__fwd/hash.h",
       "__fwd/ios.h",
       "__fwd/istream.h",