daemon: Fix GetCapabilities memory leak

[64kramsystem]

Summary

• pass the compile-time capability list directly to the generated D-Bus
  completion helper
• remove the temporary GVariantBuilder, GVariant, and duplicated string
  vector
• preserve all eight capabilities and their existing order

Real-world report

This leak was found in a real MATE desktop session after rebuilding Ubuntu's
mate-notification-daemon 1.26.1 package with AddressSanitizer and
UndefinedBehaviorSanitizer. A normal client call to
org.freedesktop.Notifications.GetCapabilities produced a 64-byte direct leak
for the duplicated pointer vector and 72 indirect bytes for its seven strings.

I reproduced the same ownership bug on current upstream master using an
ASan/UBSan build, Xvfb, and a private D-Bus session. Five
GetCapabilities calls leaked 156 bytes each: a 72-byte vector and 84 bytes
across eight strings. The isolated run produced only these two leak records:

Direct leak of 360 byte(s) in 5 object(s) allocated from:
    #2 g_variant_dup_strv
    #3 notify_daemon_get_capabilities src/daemon/daemon.c:1963

Indirect leak of 420 byte(s) in 40 object(s) allocated from:
    #3 g_variant_dup_strv
    #4 notify_daemon_get_capabilities src/daemon/daemon.c:1963

SUMMARY: AddressSanitizer: 780 byte(s) leaked in 45 allocation(s).

g_variant_dup_strv() allocates both the NULL-terminated vector and each
string. The generated completion helper marshals its input with
g_variant_new ("(^as)", return_caps), which copies the contents into the
outgoing D-Bus value and does not take ownership of the input. The duplicated
vector therefore had no owner after the helper returned.

The capability names are compile-time constants, so this change passes a
static, NULL-terminated vector directly to the helper instead.

Verification

• built unmodified master with ASan/UBSan and reproduced 780 leaked bytes
  across five calls
• rebuilt the patched source with identical sanitizer flags and repeated the
  same five calls; the g_variant_dup_strv() leak was absent
• captured all five gdbus call replies before and after; their direct diff was
  empty
• configured with --enable-compile-warnings=maximum and built the patched
  daemon normally