prboom, oprofile: Add explicit <foo>_SITE statements

The default <foo>_SITE value that pkg-generic sets when the .mk file
doesn't declare it is using BR2_SOURCEFORGE_MIRROR and therefore isn't
consistent with the usage of downloads.sourceforge.net we have
generalized for all drivers.

Since the downloads.sourceforge.net URLs are inherently specific to
each package, we can no longer provide a default in pkg-generic.mk,
and therefore each package *must* declare its own <foo>_SITE value.

This patch fixes the only two packages that were lacking a <foo>_SITE
value.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>