add missing <cstdint> for uint16_t

This fixes build problems with gcc 13 snapshot [1]

Fixes | include/apt-pkg/pkgcache.h:257:23: warning: cast from 'char*' to 'const uint16_t*' {aka 'const short unsigned int*'} increases required alignment of target type [-Wcast-align] | 257 | uint16_t len = reinterpret_cast<const uint16_t>(name - sizeof(uint16_t)); | | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[1] https://www.gnu.org/software/gcc/gcc-13/porting_to.html

Signed-off-by: Khem Raj raj.khem@gmail.com