Fix C89 compatibility issues in menu_vfs_browser.c

Move all variable declarations to function start for C89 compliance:
- const char *name
- bool is_dir
- uint64_t size
- char full_path[PATH_MAX_LENGTH]
- size_t new_capacity
- char **new_names
- bool *new_is_dir
- uint64_t *new_sizes

All variables are now declared at the beginning of vfs_browser_read_dir()
function instead of inside while loop or if blocks.

Fixes PR libretro#18827 CI build failures on linux-c89, MSYS2, and MSVC.

Author: 龙虾机器人