util/zlib-extra: de-inline gzerror_str

util/zlib-extra: de-inline gzerror_str

It turns out that putting inline functions in C header files is not a
good idea, and can cause linking problems if the compiler decides not
to inline them.  In principle this is solvable by using a "static
inline" declaration, but this potentially makes a copy in every
compilation unit. Since we don't actually care about the performance
of this function, just use a non-inline function.