[gn build] Port 6c4267f

Author: aeubanks

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

@@ -307,6 +307,7 @@ if (current_toolchain == default_toolchain) {
       "__bit/rotate.h",
       "__bit_reference",
       "__charconv/chars_format.h",
+      "__charconv/from_chars_floating_point.h",
       "__charconv/from_chars_integral.h",
       "__charconv/from_chars_result.h",
       "__charconv/tables.h",

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

@@ -133,6 +133,7 @@ cxx_sources = [
   "include/apple_availability.h",
   "include/atomic_support.h",
   "include/config_elast.h",
+  "include/from_chars_floating_point.h",
   "include/refstring.h",
   "include/ryu/common.h",
   "include/ryu/d2fixed.h",