docs/manual: Add version number to perl in prerequisites

Building host-bison needs perl 5.8.7+, as it uses the "-f" option
for site customization scripts. This feature was added in 5.8.7.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>