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

include/mimalloc.h

@@ -69,7 +69,6 @@ terms of the MIT license. A copy of the license can be found in the file
 // Includes
 // ------------------------------------------------------
 
-#include <stdlib.h>     // size_t, malloc etc.
 #include <stdbool.h>    // bool
 #include <stdio.h>      // FILE