include: make self-sufficient

This was using the LY_ERR which lives in log.h without including
anything.
diff --git a/src/parser_schema.h b/src/parser_schema.h
index bddb300..0c392d3 100644
--- a/src/parser_schema.h
+++ b/src/parser_schema.h
@@ -15,6 +15,8 @@
 #ifndef LY_PARSER_SCHEMA_H_
 #define LY_PARSER_SCHEMA_H_
 
+#include "log.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif