Fix compile error from variable type declaration

[p51d78th]

Compiling current code shows this error on macOS & Linux Mint. Reverting the one style line change fixes on both.

`src/proxguiqt.cpp: In member function ‘virtual void Plot::paintEvent(QPaintEvent*)’: src/proxguiqt.cpp:866:47: error: format ‘%zi’ expects argument of type ‘signed size_t’, but argument 6 has type ‘uint32_t’ {aka ‘unsigned int’} [-Werror=format=] 866 | snprintf(str, sizeof(str), "@%u..%u dt=%zi %szoom=%2.3f CursorAPos=%zu CursorBPos=%zu GridX=%lf

...

make[1]: [Makefile:1002: obj/proxguiqt.o] Error 1 make: [Makefile:177: client/all] Error 2`

[github-actions[bot]]

You are welcome to add an entry to the CHANGELOG.md as well

[iceman1001]

I will merge it now since people seem upset... :)