ptgen: fix protective MBR partition size

ptgen: fix protective MBR partition size

Creating gpt partition with ptgen results in invalid PMBR.

Steps to reproduce:
1) compile ptgen with WANT_ALTERNATE_PTABLE

   gcc -Wall -DWANT_ALTERNATE_PTABLE -o ptgen ptgen.c cyg_crc32.c

2) Create an image with ptgen

   ./ptgen -g -o s.img -p 509m

3) Investigate an image with /sbin/fdisk

   /sbin/fdisk s.img

4) fdisk reports GPT PMBR size mismatch

   Welcome to fdisk (util-linux 2.40.2).
   Changes will remain in memory only, until you decide to write them.
   Be careful before using the write command.

   GPT PMBR size mismatch (1042497 != 1042498) will be corrected by write.

   Command (m for help):

Signed-off-by: Mikhail Kshevetskiy <[email protected]>