bitcalc: Bumped the required GCC version for compiler warnings

Version 4.8 was high enough when compiling for x86, but not for Arm.
Not sure whether 4.9 will actually help since I do not use this version,
but bumping it to 4.9 until a compiler fail shows that this was too low.

Signed-off-by: Mats Liljegren <mats.liljegren@enea.com>