stdint, read-file: fix missing SIZE_MAX on Android (tiny change)

This is basically one of the options Bruno Haible proposed in:
http://lists.gnu.org/archive/html/bug-gnulib/2012-01/msg00282.html
* lib/sys_types.in.h (_GL_INCLUDING_UNISTD_H): New macro.
* lib/stdint.in.h: Use it.
* modules/stdint (Depends-on): Add sys_types.
maint: fix ChangeLog to match commit record