gcc/common/configure-flags.nix: fix comment

On a native (host==build==target) build, `${with_build_sysroot}` can
be omitted.  In that case, the line modified by this commit is
correct.

However, if `--with-build-sysroot=` is passed to `./configure`, then
`${with_build_sysroot}` will be used, similar to the cross case, and
the headers will be taken from:

  ${with_build_sysroot}${native_system_header_dir}

See:

  https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/configure.ac;h=62bc908b991ff0b700225aef32743cd3d7c4daaf;hb=HEAD#l2439

This commit adjusts the comment to account for the case where
`--with-build-sysroot` is passed to a native compiler.

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -119,7 +119,7 @@ let
       #  or ${with_sysroot}${native_system_header_dir}
       # While native build (build == host == target) uses passed headers
       # path as is:
-      #    ${native_system_header_dir}
+      #    ${with_build_sysroot}${native_system_header_dir}
       #
       # Nixpkgs uses flat directory structure for both native and cross
       # cases. As a result libc headers don't get found for cross case