added alarm-symbolic.svg file to gresources.xml

In the header bar of clocks application the icon of Alarm stackpage wasn't displaying properly as in window.ui the icon we were using was alarm-symbolic.svg while in gresources it wasn't there . Btw if we meant to use alarm-centered-symbolic.svg icon in header bar of window then I will edit window.ui file . Would be great if this merge request got accepted this is my first merge request! . If for reasons this could not be merged then , I would be happy to get the reason . **Closes Issue #217 (closed) **