Commit 40016a6a92 ("libc/minimal: Use a sys_heap for the malloc
implementation") replaced sys_mem_pool_alloc() with sys_heap_alloc().
The problem is that those aren't equivalent. While the former did
guard against concurrent usage, the later doesn't.
Add the same locking around sys_heap_alloc() that used to be implicit
with sys_mem_pool_alloc().
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>