Fix build dependencies of symtab2gb

Depending on the exact order of building directories, Makefile-based
builds of symtab2gb would sometimes fail as json.a had not been built
(and no rule for building it was present).

Author: tautschnig

src/Makefile

@@ -88,7 +88,8 @@ goto-cc.dir: languages goto-programs.dir linking.dir
 
 memory-analyzer.dir: util.dir goto-programs.dir ansi-c.dir
 
-symtab2gb.dir: util.dir goto-programs.dir json-symtab-language.dir
+symtab2gb.dir: util.dir goto-programs.dir langapi.dir linking.dir \
+               json.dir json-symtab-language.dir
 
 # building for a particular directory