BaseTools/GenFfs: Enlarge the size of 'AlignmentBuffer'

BaseTools/GenFfs: Enlarge the size of 'AlignmentBuffer'

As a workaround for the static code checkers, enlarge the size of the
string buffer 'AlignmentBuffer' so that it can hold all the digits of an
unsigned 32-bit integer plus the size unit character (e.g. 'M' & 'K').

Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>