Fix bogus "extern int errno;" in back branches, per Andrew Dunstan.

contrib/seg/segparse.y

@@ -3,6 +3,7 @@
   
 #include "postgres.h"
 
+#include <errno.h>
 #include <math.h>
 
 #include "segdata.h"
@@ -14,7 +15,6 @@
 #undef yylex                  /* falure to redefine yylex will result in calling the */
 #define yylex seg_yylex       /* wrong scanner when running inside postgres backend  */
 
-  extern int errno;
   extern int yylex();           /* defined as seg_yylex in segscan.c */
   extern int significant_digits( char *str );    /* defined in seg.c */