Also extract translatable strings in header files

[echoix]

a string surrounded with _() will produce a localised string (with gettext)

With the latest change to Weblate, I do not know how string extraction is done, but at least in our Make file strings from headers are not extracted

grass/locale/Makefile

Line 34 in 559e088

LIB_POTFILES = find ../lib \( -name "*.c" -o -name "*.py" \) | xargs grep -l "_(\"\|n_(\""

Originally posted by @marisn in #3970 (comment)

I don't know if there are some counter-indications for this, but this was discussed in the PR #3970 when a suggestion included a translatable string in a C++ header file. It is simply filing the issue before it gets lost.