[C][Client] Add gcc predefined macros to reserved keywords

[ityuhui]

By default, gcc has a predefined macro “linux”

$ gcc -dM -E - < /dev/null | grep linux
#define __linux 1
#define __gnu_linux__ 1
#define linux 1                    <---------------------- here !
#define __linux__ 1

So if the source code contains "linux", the compilation will fail.
e.g.

/home/runner/work/c/c/kubernetes/model/v1_container_user.h:23:39: error: expected identifier or ‘(’ before numeric constant
   23 |     struct v1_linux_container_user_t *linux; //model
      |                                       ^~~~~

This PR adds "linux" to the reserved keywords to escape "linux" to "_linux" to avoid compilation failure.

@wing328 @zhemant @michelealbano

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[zhemant]

Thanks for the changes, LGTM