package/refpolicy: ensure REFPOLICY_EXTRA_MODULES_DIRS is empty when it should be

When both BR2_REFPOLICY_EXTRA_MODULES_DIRS and
PACKAGES_SELINUX_EXTRA_MODULES_DIRS are empty, we expect
REFPOLICY_EXTRA_MODULES_DIRS to also be empty. However, due to spaces,
this is not the case. This commit adds a $(strip ...) call to ensure
it is the case.

Thanks to this, the check on whether REFPOLICY_EXTRA_MODULES_DIRS is
empty later on will really work as it should.

Reported-by: Antoine Ténart <antoine.tenart@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>