fix cgo build following arch

The build on Mac were failing (if not using cgo) because go looked to ".c" files and figure that you may need cgo, which is actually only used for linux. ``` $ make build go build -o ./blup ./cmd/blup package tangled.org/evan.jarrett.net/blup/cmd/blup imports tangled.org/evan.jarrett.net/blup/internal/clipboard: C source files not allowed when not using cgo or SWIG: clipboard_wayland.c wlr-data-control.c xdg-shell.c make: *** [build] Error 1 ``` Adding the directive `//go:build linux` in head to the .c files is enough to solve it. Another solution would be rename some files to `_linux` but I dont want to be renaming the files Signed-off-by: Fabricio Silva <hi@fabricio.dev>