pcre: always enable the 8-bits version

Commit 7154798a (pcre: add support for 16 bits and 32 bits
variants), added in preparation to the introduction of the Qt5
packages, could break other packages that use pcre.

Even though by default the pcre 8 bits library is selected if no other
variant is selected, a randpackageconfig, or an user, could
potentially enable the 16 bits variant even if a package requires the
8 bits variant.

This has caused a number of build breakage on packages like ngrep,
cegui, nmap and others in the autobuilders.

In older to solve this, we simply unconditionally build the 8-bits
variant. Since each variant is only ~90k in size, it's probably not
worth the effort giving the possibility of building the 16 bits
variant only.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>