Remove function pointer casts when calling xdg_run_command_on_dirs().

Merged Daniel Cheng requested to merge dcheng/xdgmime:moo into master

The function pointer casts silence the compiler and allow the code to build (and even run in the typical case). However, when building with control flow integrity checks, the runtime (rightfully) complains about calling a function via a mismatched function pointer type.

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