readmmap.c: fix building with MUSL libc

glibc includes sys/types.h from stdlib.h. This is not the case for MUSL,
so explicitly include it. Fixes usage of uint_t.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Zach Dykstra <dykstra.zachary@gmail.com>
Closes #15130