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.

util/zlib-extra.c

@@ -85,3 +85,10 @@ gz_error_string (util_status_t status, gzFile file)
     else
 	return util_error_string (status);
 }
+
+const char *
+gzerror_str(gzFile file)
+{
+    int dummy;
+    return gzerror (file, &dummy);
+}

util/zlib-extra.h

@@ -29,8 +29,8 @@ gz_error_string (util_status_t status, gzFile stream);
 
 /* Call gzerror with a dummy errno argument, the docs don't promise to
  * support the NULL case */
-inline const char *
-gzerror_str(gzFile file) { int dummy; return gzerror (file, &dummy); }
+const char *
+gzerror_str(gzFile file);
 
 #ifdef __cplusplus
 }