libhugetlbfs: avoid using keyword restrict as var name

When compile with gcc 5.x, libhugetlbfs fails with:

| hugeutils.c: In function '__lh_hugetlbfs_setup_env':
| hugeutils.c:304:40: error: expected identifier or '(' before restrict'
|    char *p, *tok, *exe, buf[MAX_EXE+1], restrict[MAX_EXE];
|                                         ^

Rename variable to avoid using keyword restrict as variable name to fix it.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>