Remove bogus extern for 'descriptors' variable, which is in fact static.

Some compilers object to seeing extern and later static ...

src/interfaces/ecpg/preproc/extern.h

@@ -36,7 +36,6 @@ extern struct when when_error,
 			when_nf,
 			when_warn;
 extern struct ECPGstruct_member *struct_member_list[STRUCT_DEPTH];
-extern struct descriptor *descriptors;
 
 /* functions */