Enforce strict include-what-you-use policy

The include-what-you-use (IWYU) policy is beneficial to faster
compilation and fewer recompilations. Many build tools, such as GNU make,
provide a mechanism for automatically figuring out what .h files a .cc
file depends on. These mechanisms typically look at #include lines. When
unnecessary #includes are listed, the build system is more likely to
recompile in cases where it is not necessary.

With the enforcement, header file <include/mimalloc.h> no longer
includes <stdlib.h>.

Reference:
https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/WhyIWYU.md

src/init.c

@@ -7,7 +7,8 @@ terms of the MIT license. A copy of the license can be found in the file
 #include "mimalloc.h"
 #include "mimalloc-internal.h"
 
-#include <string.h>  // memcpy
+#include <string.h>  // memcpy, memset
+#include <stdlib.h>  // atexit
 
 // Empty page used to initialize the small free pages array
 const mi_page_t _mi_page_empty = {