BLD: add back stdlib.h include in pcg64.h

This seems to have been removed by accident in numpy#21887
stdlib.h is required for _rotr64() further down.

Fixes:
  error: call to undeclared function '_rotr64'; ISO C99 and later do not support
  implicit function declarations [-Wimplicit-function-declaration]

Author: lazka

numpy/random/src/pcg64/pcg64.h

@@ -52,8 +52,11 @@
 
 #include <inttypes.h>
 
-#if defined(_WIN32) && !defined (__MINGW32__)
+#ifdef _WIN32
 #include <stdlib.h>
+#endif
+
+#if defined(_WIN32) && !defined (__MINGW32__)
 #define inline __forceinline
 #endif