Fix small leak in startui.c ensureDotFontForgeIsSetup()

Another use of the recent routine gutils/fsys.c :: getFontForgeUserDir()
that requires a free() of the newly allocated string after use.

Tested and leak now gone from valgrind reports.
Merge pull request #1599 from tshinnic/leak_getuserdir_startuic_1

Fix small leak in startui.c ensureDotFontForgeIsSetup()