libgfs2: Fix strict-aliasing warning in lgfs2_rgrp_bitbuf_alloc
    
    Enabling strict aliasing warnings produces:
    
      rgrp.c: In function ‘lgfs2_rgrp_bitbuf_alloc’:
      rgrp.c:121:37: warning: dereferencing type-punned pointer might
      break strict-aliasing rules [-Wstrict-aliasing]
    
    Use a void pointer for the buffer allocation to clear up the warning.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>