Fix build failure due to libc++ using libc functions

Include the "alloc-private.h" header after the C++ standard library
headers. This sidesteps build failures caused by implementations of
std::map and std::string which use libc memory allocation functions
in expanded templates after they have been marked with the "poison"
pragma.

Fixes #115

(cherry picked from commit d77bc02)

Author: aperezdc

src/pasteboard-generic.cpp

@@ -26,12 +26,15 @@
 
 #include "pasteboard-private.h"
 
-#include "alloc-private.h"
-#include <cstdlib>
-#include <cstring>
 #include <map>
 #include <string>
 
+// We need to include this header last, in order to avoid template expansions
+// from the C++ standard library happening after it forbids usage of the libc
+// memory functions.
+#include "alloc-private.h"
+#include <cstring>
+
 namespace Generic {
 using Pasteboard = std::map<std::string, std::string>;
 }