vg_common_json: use %zu for size_t

vg_common_json: use %zu for size_t

Summary:
Fixes a compiler warning
```
../src/static_libs/vg_common/vg_common_json.c: In function '_get_key_val':
../src/static_libs/vg_common/vg_common_json.c:16:34: warning: format '%ld' expects argument of type 'long int', but argument 4 has type 'unsigned int' [-Wformat=]
```

Test Plan: ninja build

Reviewers: Hermet, jsuya

Reviewed By: jsuya

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D11327