make function inline so that the symbol does not get defined several …

…times.

According to the error message:
/home/runner/work/IOHexperimenter/IOHexperimenter/include/ioh/common/file.hpp:115: multiple definition of `ioh::common::file::utils::get_static_root[abi:cxx11]()'; CMakeFiles/test_ioh.dir/cpp/common/test_common.cpp.o:/home/runner/work/IOHexperimenter/IOHexperimenter/include/ioh/common/file.hpp:115: first defined here
There are actually at least .cpp files that are merged together before compilation, that both include this file.hpp.

include/ioh/common/file.hpp

@@ -111,7 +111,7 @@ namespace ioh::common::file
          *
          * @return fs::path the absolute path of IOHexperimenter/static
          */
-        fs::path get_static_root()
+        inline fs::path get_static_root()
         {
             // Print the contents of the current, previous, and the one before previous directories
             print_directory_contents(fs::current_path());