Fix implicit declaration of function strerror

[ryandesign]

I'm working on getting this to compile on Linux. On Ubuntu Server 20.04 with gcc 9.3.0, I noticed this warning:

/home/rschmidt/xserve-frontpanel/cpu_usage.c:22:50: warning: implicit declaration of function ‘strerror’; did you mean ‘perror’? [-Wimplicit-function-declaration]
   22 |         printf("ERROR fetching hw.packages: %s", strerror(errno));
      |                                                  ^~~~~~~~
      |                                                  perror

strerror(3) tells us that it is defined in <string.h> so the fix is to include that header.

(I don't see this warning on macOS 10.13.6 with clang. Presumably there, <string.h> got included automatically by one of the other headers.)

[ryandesign]

I have several other small unconnected fixes I'd like to submit. Would you prefer a separate pull request for each? Or all together in one PR with separate commits? Or include the small fixes with my work on adding Linux support (which is where they currently are)?

[castvoid]

Hey @ryandesign! Thanks for submitting this fix :)

Sorry for the delay in reviewing. This looks good to me, so I'll merge your PRs now.