gcc-compat.h: Remove _static_inline_ macro.

The only purpose of this macro is to have a way to include static
inline functions into the doxygen source code documentation (but omit
normal static functions in .c files).  Since dss does not use doxygen,
the macro is pointless.

Remove the equally pointless documentation of dss_rename(), one
of the two users of _static_inline_, while converting it to plain
static inline.