Add missing extern.h file

src/interfaces/ecpg/preproc/extern.h

+/* variables */
+
+extern int debugging, braces_open;
+extern char * yytext;
+extern int yylineno, yyleng;
+extern FILE *yyin, *yyout;
+
+
+/* functions */
+
+extern void lex_init(void);
+extern char * input_filename;
+extern int yyparse(void);