Update libqt5core5a Provides to qtbase-abi-5-15-1

The current package in all channels of the Neon repositories has fallen out of sync with current Debian packaging. This MR updates the value for the Provides key in the libqt5core5a Binary stanza to match the upstream version being packaged. It resolves the broken package issues for all of the packages in the python3-pyqt5-* family which all depend directly or indirectly on the virtual package qtbase-abi-5-15-1 to be present on the system prior to installation/upgrade.