use freestanding headers

core/mem/pmap.c

@@ -4,7 +4,7 @@
 #include <zinnia/spin.h>
 #include <zinnia/vma.h>
 #include <config.h>
-#include <string.h>
+#include <zinnia/string.h>
 
 // TODO: For now, we assume that we can do generic PTE lookups on every arch that has a MMU.
 static_assert(CONFIG_HAVE_MMU_GENERIC);