x86/build: Use the proper name CONFIG_FW_LOADER

Commit in Fixes intends to add the expression regex only when FW_LOADER
is enabled - not FW_LOADER_BUILTIN. Latter is a leftover from a previous
patchset and not a valid config item.

So, adjust the condition to the actual name of the config.

  [ bp: Cleanup commit message. ]

Fixes: c8dcf655

 ("x86/build: Tuck away built-in firmware under FW_LOADER")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20211229111553.5846-1-lukas.bulwahn@gmail.com