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>

contrib/seg/segscan.l

@@ -3,6 +3,8 @@
  * A scanner for EMP-style numeric ranges
  */
 
+/* LCOV_EXCL_START */
+
 /* No reason to constrain amount of data slurped */
 #define YY_READ_BUF_SIZE 16777216
 
@@ -51,6 +53,8 @@ float        ({integer}|{real})([eE]{integer})?
 
 %%
 
+/* LCOV_EXCL_STOP */
+
 void
 yyerror(SEG *result, const char *message)
 {