[util] Enable parser debugging for cexpr

Not actually turned on in the code, but cexpr_yydebug is present.

libs/util/cexpr-lex.l

@@ -50,6 +50,8 @@
 #include "QF/qfplist.h"
 #include "QF/sys.h"
 
+#define CEXPR_YYDEBUG 1
+
 #include "QF/cexpr.h"
 #include "libs/util/cexpr-parse.h"
 

libs/util/cexpr-parse.y

@@ -27,6 +27,7 @@
 %define api.prefix {cexpr_yy}
 %define api.pure full
 %define api.push-pull push
+%define parse.trace
 %parse-param {void *scanner} {exprctx_t *context}
 
 %{