Exclude flex-generated code from coverage testing

Flex generates a lot of functions that are not actually used.  In order
to avoid coverage figures being ruined by that, mark up the part of the
.l files where the generated code appears by lcov exclusion markers.
That way, lcov will typically only reported on coverage for the .l file,
which is under our control, but not for the .c file.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

src/fe_utils/psqlscan.l

@@ -71,6 +71,8 @@ typedef int YYSTYPE;
 extern int	psql_yyget_column(yyscan_t yyscanner);
 extern void psql_yyset_column(int column_no, yyscan_t yyscanner);
 
+/* LCOV_EXCL_START */
+
 %}
 
 %option reentrant
@@ -899,6 +901,8 @@ other			.
 
 %%
 
+/* LCOV_EXCL_STOP */
+
 /*
  * Create a lexer working state struct.
  *