gitk: fix msgfmt being required

While the Meson build instructions already handle the case where msgfmt wasn't found, we forgot to mark the dependency itself as optional. This causes an error in case the executable could not be found: Project name: gitk Project version: undefined Program sh found: YES (C:\Program Files\Git\bin\sh.EXE) Program wish found: YES (C:\Program Files\Git\mingw64\bin\wish.EXE) Program chmod found: YES (C:\Program Files\Git\usr\bin\chmod.EXE) Program mv found: YES (C:\Program Files\Git\usr\bin\mv.EXE) Program sed found: YES (C:\Program Files\Git\usr\bin\sed.EXE) Program msgfmt found: NO subprojects\gitk\meson.build:28:3: ERROR: Program 'msgfmt' not found or not executable Fix the issue by adding the `required: false` parameter. Signed-off-by: Patrick Steinhardt <ps@pks.im>