cl/gcc: fix compatibility with host gcc-14.x

This commit fixes the following build-time error:
```
cl/gcc/clplug.c: In function 'bool error_detected()':
cl/gcc/clplug.c:480:52: error: invalid types '<unresolved overloaded function type>[diagnostic_t]' for array subscript
  480 |     return global_dc && global_dc->diagnostic_count[DK_ERROR];
      |                                                    ^
```

Related: #92
Resolves: #93

cl/gcc/clplug.c

@@ -477,7 +477,7 @@ static void var_db_insert(var_db_t db, struct cl_var *var)
 
 static bool error_detected(void)
 {
-    return global_dc && global_dc->diagnostic_count[DK_ERROR];
+    return global_dc && errorcount;
 }
 
 static void read_gcc_location(struct cl_loc *loc, location_t gcc_loc)