fix compile warnings

[IanoNjuguna]

Added #include <string.h> to resolve implicit function declaration warnings.

The code was using strncmp() and strchr() functions without including the necessary header, and it caused these compilation warnings:

• warning: implicit declaration of function 'strncmp'
• warning: implicit declaration of function 'strchr'
• warning: incompatible implicit declaration of built-in function

Testing

• Compiled with gcc -Wall -Wextra detect-cpu.c -o detect-cpu
• All previous warnings are now resolved
• Functionality works as intended