otautil: Clean up obsolete includes.

external/zlib and external/safe-iop/include were used by Zip.c, which
became obsolete after [1].

Removing
 - <assert.h>: we no longer call assert(3);
 - <limits.h>: we no longer use PATH_MAX;
 - <stdio.h>: we don't need FILE anymore;
 - <stdlib.h>: no more malloc(3)/calloc(3);
 - <unistd.h>: was once needed for lseek(2);
 - <string.h>: no more memset(3).

Adding
 - <stdint.h> for SIZE_MAX.

[1] commit 8cf5c8f6: "Replace minzip
with libziparchive".

Test: `mmma bootable/recovery`
Change-Id: I349e909ba83d0f6ade5f92856ce3934f29fdca6c