Fix timegm conflicting types error on AIX

[jeffreyparker]

Summary

• Include <time.h> before declaring the timegm fallback prototype in acconfig.h
• Without this include, struct tm is an incomplete type local to the function declaration
• When config.h is included multiple times (from pam_duo.c:12 and again via pam_extra.h:9), each inclusion creates a different incomplete struct tm type, causing "conflicting types for 'timegm'" compilation errors

This follows the same pattern as other compat functions in acconfig.h (e.g., asprintf includes <stdarg.h>, getgrouplist includes <grp.h>).

Fixes #356

Test plan

• Build on AIX 7.3 with OpenSSL 3.0 (customer environment that reported the issue)
• Verify existing CI tests pass