core: fix memory leak in manager_run_generators()

core: fix memory leak in manager_run_generators()

If systemd is built with GCC address sanitizer or leak sanitizer
the following memory leak ocurs:

May 12 02:02:46 linux.site systemd[326]: =================================================================
May 12 02:02:46 linux.site systemd[326]: ==326==ERROR: LeakSanitizer: detected memory leaks
May 12 02:02:46 linux.site systemd[326]: Direct leak of 101 byte(s) in 3 object(s) allocated from:
May 12 02:02:46 linux.site systemd[326]: #0 0x7fd1f504993f in strdup (/usr/lib64/libasan.so.2+0x6293f)
May 12 02:02:46 linux.site systemd[326]: #1 0x55d6ffac5336 in strv_new_ap src/shared/strv.c:163
May 12 02:02:46 linux.site systemd[326]: #2 0x55d6ffac56a9 in strv_new src/shared/strv.c:185
May 12 02:02:46 linux.site systemd[326]: #3 0x55d6ffa80272 in generator_paths src/shared/path-lookup.c:223
May 12 02:02:46 linux.site systemd[326]: #4 0x55d6ff9bdb0f in manager_run_generators src/core/manager.c:2828
May 12 02:02:46 linux.site systemd[326]: #5 0x55d6ff9b1a10 in manager_startup src/core/manager.c:1121
May 12 02:02:46 linux.site systemd[326]: #6 0x55d6ff9a78e3 in main src/core/main.c:1667
May 12 02:02:46 linux.site systemd[326]: #7 0x7fd1f394e8c4 in __libc_start_main (/lib64/libc.so.6+0x208c4)
May 12 02:02:46 linux.site systemd[326]: Direct leak of 29 byte(s) in 1 object(s) allocated from:
May 12 02:02:46 linux.site systemd[326]: #0 0x7fd1f504993f in strdup (/usr/lib64/libasan.so.2+0x6293f)
May 12 02:02:46 linux.site systemd[326]: #1 0x55d6ffac5288 in strv_new_ap src/shared/strv.c:152
May 12 02:02:46 linux.site systemd[326]: #2 0x55d6ffac56a9 in strv_new src/shared/strv.c:185
May 12 02:02:46 linux.site systemd[326]: #3 0x55d6ffa80272 in generator_paths src/shared/path-lookup.c:223
May 12 02:02:46 linux.site systemd[326]: #4 0x55d6ff9bdb0f in manager_run_generators src/core/manager.c:2828
May 12 02:02:46 linux.site systemd[326]: #5 0x55d6ff9b1a10 in manager_startup src/core/manager.c:1121
May 12 02:02:46 linux.site systemd[326]: #6 0x55d6ff9a78e3 in main src/core/main.c:1667
May 12 02:02:46 linux.site systemd[326]: #7 0x7fd1f394e8c4 in __libc_start_main (/lib64/libc.so.6+0x208c4)
May 12 02:02:46 linux.site systemd[326]: SUMMARY: AddressSanitizer: 130 byte(s) leaked in 4 allocation(s).

There is a leak due to the the use of cleanup_free instead _cleanup_strv_free_