Fix errors when generating flameshot.icons

[sryze]

On macOS when you configure Flameshot with CMake it produces errors during generation of the iconset file:

mkdir build/xcode
cd build/xcode
cmake ../../ -G Xcode

Warning: ../data/img/app/flameshot.png not a valid file - skipping
Warning: ../data/img/app/flameshot.png not a valid file - skipping
Warning: ../data/img/app/flameshot.png not a valid file - skipping
Warning: ../data/img/app/flameshot.png not a valid file - skipping
Warning: ../data/img/app/flameshot.png not a valid file - skipping
Warning: ../data/img/app/flameshot.png not a valid file - skipping
Warning: ../data/img/app/flameshot.png not a valid file - skipping
Warning: ../data/img/app/org.flameshot.Flameshot-1024.png not a valid file - skipping
Warning: ../data/img/app/org.flameshot.Flameshot-1024.png not a valid file - skipping
Warning: ../data/img/app/org.flameshot.Flameshot-1024.png not a valid file - skipping
Warning: ../data/img/app/org.flameshot.Flameshot-1024.png not a valid file - skipping
Warning: ../data/img/app/org.flameshot.Flameshot-1024.png not a valid file - skipping
flameshot.iconset:Failed to generate ICNS.
cp: ../packaging/macos/flameshot.icns: No such file or directory
-- Found Git: /usr/bin/git (found version "2.30.1 (Apple Git-130)")
git found: /usr/bin/git in version     2.30.1 (Apple Git-130)
FLAMESHOT_GIT_HASH: a1e6a2f
-- Configuring done
CMake Error at src/CMakeLists.txt:55 (add_executable):
  Cannot find source file:

    /Users/szx/Sources/flameshot/build/xcode/flameshot.icns

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm .h
  .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90 .f95 .f03 .hip .ispc

Solution: instead of a relative path ../data/ use ${CMAKE_SOURCE_DIR}/data/

[veracioux]

@sryze Thanks :)