Bluetooth: tester: enable VCS volume flags notify

[Description]
PTS case: VCS/SR/SGGIT/CHA/BV-03-C fail.

[Root Cause]
VCS/SR/SGGIT/CHA/BV-03-C require Volume Flags Characteristic properties
(Read/Notify), but tester volume flags notify is disabled by default.

[Fix]
add CONFIG_BT_VCP_VOL_REND_VOL_FLAGS_NOTIFIABLE=y in
tests/bluetooth/tester/overlay-le-audio.conf

[Testing]
After modified, VCS/SR/SGGIT/CHA/BV-03-C pass with log:
Test Case Ended	VCS/SR/SGGIT/CHA/BV-03-C	PASS	24	 00:00:01.010000
12/9/2024 11:31:55.027000 AM

Signed-off-by: Chang An <chang.an_1@nxp.com>