lib/alloc.c: fix missing include

Include bitops.h to get BITS_PER_LONG and avoid errors such as

lib/alloc.c: In function mult_overflow:
lib/alloc.c:24:9: error: right shift count >= width of type
[-Werror=shift-count-overflow]
   24 |  if ((a >> 32) && (b >> 32))
      |         ^~

Fixes: cde8415e ("lib/alloc.c: add overflow check for calloc")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>