The musl C library is very strict with header includes. In sudo_util.h id_t is used so <sys/types.h> has to be included.

This is the error message if <sys/types.h> is NOT included in sudo_util.h:

In file included from ./sudo.h:39:0,
                 from ./preserve_fds.c:32:
../include/sudo_util.h:220:14: error: unknown type name 'id_t'
 __dso_public id_t sudo_strtoid_v1(const char *str, const char *sep, char **endp, const char **errstr);
              ^
Makefile:329: recipe for target 'preserve_fds.o' failed