[gn build] Port af0d731

Author: llvmgnsyncbot

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

@@ -580,6 +580,7 @@ if (current_toolchain == default_toolchain) {
       "__math/remainder.h",
       "__math/roots.h",
       "__math/rounding_functions.h",
+      "__math/special_functions.h",
       "__math/traits.h",
       "__math/trigonometric_functions.h",
       "__mbstate_t.h",