cgen: fix bootstrapping on older macOS Catalina

Author: spytheman

vlib/v/gen/c/cheaders.v

@@ -556,7 +556,7 @@ voidptr memdup(voidptr src, int sz);
 	#error Cygwin is not supported, please use MinGW or Visual Studio.
 #endif
 
-#if defined(__linux__) || defined(__FreeBSD__) || defined(__DragonFly__) || defined(__vinix__) || defined(__serenity__) || defined(__sun)
+#if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || defined(__DragonFly__) || defined(__vinix__) || defined(__serenity__) || defined(__sun)
 	#include <sys/types.h>
 	#include <sys/wait.h> // os__wait uses wait on nix
 #endif