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>

docs/manual/prerequisite.txt

@@ -33,7 +33,7 @@ Mandatory packages
 ** +patch+
 ** +gzip+
 ** +bzip2+
-** +perl+
+** +perl+ (version 5.8.7 or any later)
 ** +tar+
 ** +cpio+
 ** +python+ (version 2.6 or 2.7)