firmware loader: fix compile warning

The commit ddf1f064("firmware loader: fix build failure
with !CONFIG_FW_LOADER_USER_HELPER") introduces the below
warning:

drivers/base/firmware_class.c:921:13: warning:
'kill_requests_without_uevent' defined but not used [-Wunused-function]

So fix it by defining kill_requests_without_uevent() only if
CONFIG_PM_SLEEP is set.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>