treewide: apply formatting changes with clang-format

treewide: apply formatting changes with clang-format

Use linux kernel's .clang-format file to automatically improve the coding
style of libgpiod's C code base. We don't import the file into the
repository as it's not perfect and certain converted fragments were
rolled back because they looked better before the conversion.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>