Draft: Fix typo by renaming constant from PRIO_PROGRESS to PRIO_PROCESS in sys_resource header.

During a test compilation of ZSTD I discovered that it tries to use a constant from sys/resource.h named PRIO_PROCESS, but relibc had this constant defined as PRIO_PROGRESS with same value and context. This change is a fix of this typo, I couldn't find anything else in the codebase using this constant with the old name so existing sources shouldn't be affected, relibc and an OS image was built with the change successfully. Note: this MR was superseded by MR 459 because I accidentally made a mistake with rebase.