link time optimization requires all functions named the same to actually
operate the same. I am pretty sure this fixes it without breaking
anything.
Fixes: https://github.com/RfidResearchGroup/proxmark3/issues/2058
src/proxgui.h:35:6: error: type of ‘ShowPictureWindow’ does not match original declaration [-Werror=lto-type-mismatch]
35 | void ShowPictureWindow(uint8_t *data, int len);
| ^
src/guidummy.cpp:33:17: note: type mismatch in parameter 2
33 | extern "C" void ShowPictureWindow(char *fn) {
| ^
src/guidummy.cpp:33:17: note: type ‘void’ should match type ‘int’
src/guidummy.cpp:33:17: note: ‘ShowPictureWindow’ was previously declared here
