meson: Multi-lines string should use ''' with meson (#8225)

This breaks with latest version of meson:
https://hephaistos.lpp.polytechnique.fr/teamcity/viewLog.html?buildId=11653&buildTypeId=mesonbuild_SystemdNightly&tab=buildLog&state=1059#_state=1059&guest=true

(cherry picked from commit 12873b6c)