timer/linux: Add missing header

[jjhursey]

Missing header found by MTT using either GNU or XL compilers and the following compiler option:
-Werror,-Wimplicit-function-declaration

timer_linux_component.c:167:9: error: implicit declaration of function 'opal_show_help' is invalid
in C99 [-Werror,-Wimplicit-function-declaration]
        opal_show_help("help-opal-timer-linux.txt", "monotonic not supported", true);
        ^1 warning and 1 error generated.
Error while processing timer_linux_component.c.

This was fixed as part of a larger commit on master, so there is no commit to cherry-pick.

[jsquyres]

@hppritcha Seems very low risk / no brainer.

[jjhursey]

@hppritcha @jsquyres Can we merge this in? It's blocking our MTT testing on this branch.