[TF-A] Function bl31_lib_init() has external linkage but is only used in one translation unit.

Hello,

In the file bl31/bl31_main.c, there is a function named bl31_lib_init with its definition. Simultaneously, the declaration of this function is included in include/bl31/bl31.h since the function bl31_lib_init used only in one translational unit. leading to a violation when running the Coverity MISRA-C analysis for the ZynqMP. Addressing this issue need to remove the declaration of the function from the bl31.h file and making its definition static. To maintain visibility, included a static declaration in the bl31_main.c file. However, it will may impact in future modifications due to the removal of the declaration in bl31.h. Please provide your suggestions.

Regards, Nithin