doc: Note need of bc in the kernel build from 3.10 onwards

HPA did the world a favour and reduced the number of perl scripts in the
universe. However we do now need bc

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=60575
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/Changes

@@ -73,6 +73,11 @@ Perl
 You will need perl 5 and the following modules: Getopt::Long, Getopt::Std,
 File::Basename, and File::Find to build the kernel.
 
+BC
+--
+
+You will need bc to build kernels 3.10 and higher
+
 
 System utilities
 ================