fix(build): Comment out impl func dec in prefix_c99 test.

tests/prefix_c99.l calls a function before its definition from within
a dead code block. This is treated as an error in newer compilers. The
test isn't needed as the scanner's main() repeats the call.

Refs: #632

tests/prefix_c99.l

@@ -64,7 +64,7 @@
         FOOget_text( (yyscan_t )0 );
         FOOlex( (yyscan_t )0 );
         //FOOlex_destroy( (yyscan_t )0 );
-        FOOlex_init( (yyscan_t *)0 );
+        //FOOlex_init( (yyscan_t *)0 );
         //FOOset_extra( (void *)0, (yyscan_t )0 );
         FOOset_in( (FILE*)0, (yyscan_t )0 );
         FOOset_out( (FILE*)0, (yyscan_t )0 );