libical: Update to 3.0.6

Changes in 3.0.6:

    Handle both COUNT and UNTIL in RRULEs
    Fix RRULE BYDAY with INTERVAL=2 conflict
    Various fuzzification fixes
    New publicly available function:
        icaltimezone_truncate_vtimezone()
    Add option to disable building the test suite
    Built-in timezones updated to tzdata2019c

Changes in 3.0.5:

    New publicly available function:
        icalproperty_get_datetime_with_component()
    Allow reset DATE/DATE-TIME VALUE parameter for all-day events
    icalproperty_get_datetime_with_component() will use location as TZID fallback.
    New CMake option ENABLE_GTK_DOC for disabling the libical-glib developer documentation
    GObject Introspection - use $MAJOR-0 versioning
    libical-glib API is considered unstable:
        define LIBICAL_GLIB_UNSTABLE_API=1 before including <libical-glib/libical-glib.h>
    Built-in timezones updated to tzdata2019a
    De-fuzzifications and Coverity fixes

Changes in 3.0.4:

    Silently fail RSCALE recurrence clauses when RSCALE is disabled
    Fixed icalcomponent_set_comment() and icalcomponent_set_uid()
    fix FREQ=MONTHLY;BYMONTH
    Skip UTF-8 marker when parsing
    Fix parsing ? in VCF files produced by Outlook
    Fix TZID on DATE-TIME value can override time specified in UTC
    CMake discovery module for ICU uses pkg-config now
    New publicly available function:
        icalparameter_kind_is_valid()
    Built-in timezones updated to tzdata2018e

(nia)