ootstrap: Fix bootstrap with --disable-plugin [PR104176]

With --disable-plugin, bootstrap fails on x86_64-linux or probably
all other targets with:
../../gcc/opts-global.cc: In function ‘void handle_common_deferred_options()’:
../../gcc/opts-global.cc:420:62: error: unquoted option name ‘--enable-plugin’ in format [-Werror=format-diag]
  420 |           error ("plugin support is disabled; configure with --enable-plugin");
      |                                                              ^~~~~~~~~~~~~~~
../../gcc/opts-global.cc:428:62: error: unquoted option name ‘--enable-plugin’ in format [-Werror=format-diag]
  428 |           error ("plugin support is disabled; configure with --enable-plugin");
      |                                                              ^~~~~~~~~~~~~~~

The following patch fixes that.

2022-01-22  Jakub Jelinek  <jakub@redhat.com>

	PR other/104176
	* opts-global.cc (handle_common_deferred_options): Quote
	--enable-plugin in diagnostics to avoid -Werror=format-diag.