Fix build with GPSD

[wegank]

The following error was observed (NixOS/nixpkgs#259434 (comment)) when building Merkaartor with GPSD support:

error: builder for '/nix/store/b4c5hs4509g37l7y9kmb1ysd1916jjrj-merkaartor-0.19.0.drv' failed with exit code 1;
       last 10 log lines:
       > -- Checking for modules 'gdal;proj;exiv2;gpsd'
       > --   No package 'gpsd' found
       > CMake Error at /nix/store/yi235g10sp8jx939zpfli0s74154ph3v-cmake-3.26.4/share/cmake-3.26/Modules/FindPkgConfig.cmake:607 (message):
       >   A required package was not found
       > Call Stack (most recent call first):
       >   /nix/store/yi235g10sp8jx939zpfli0s74154ph3v-cmake-3.26.4/share/cmake-3.26/Modules/FindPkgConfig.cmake:829 (_pkg_check_modules_internal)
       >   CMakeLists.txt:523 (pkg_check_modules)
       >
       >
       > -- Configuring incomplete, errors occurred!

It turns out that the package is known as libgps instead.

[Krakonos]

Merged, thanks for your contribution. I'm not sure how this got through, I'm pretty sure I tested that.