Add a missing extern, to fix building with -fno-common (3.24 branch)

Martin Storsjö requested to merge mstorsjo/gtk:fno-common-3-24 into gtk-3-24

With C compilers defaulting to -fcommon, this isn't an issue, but upcoming compilers (GCC 10 and Clang 11) will default to -fno-common, ending up with duplicate definitions of this variable.

This is the same as !1631 (merged), but the master and 3.24 branches differ at this spot, thus making the backport a separate MR.

Merge request reports