Fix building on Mac OS X 10.11 or older

* Add a missing include in src/core/Alloc.cpp

On Mac OS X 10.11 with Xcode 8.2.1, building fails with

/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_security_KeePassXC/KeePassXC-devel/work/keepassxc-f726d7501ff7e8a66ae974719042f23010716595/src/core/Alloc.cpp:44:10: error: no type named 'free' in namespace 'std'
    std::free(ptr);
    ~~~~~^

Per [1], std::free() needs #include <cstdlib>. That file is included
indirectly on newer systems.

* Avoid const Signature object in src/keeshare/ShareExport.cpp

After the above issue is resolved, building fails at

/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_security_KeePassXC/KeePassXC-devel/work/keepassxc-f726d7501ff7e8a66ae974719042f23010716595/src/keeshare/ShareExport.cpp:152:29: error: default initialization of an object of const type 'const Signature' without a user-provided default constructor
            const Signature signer;
                            ^

Apparently this is related to C++ defect 253 [2]. From the code,
creating a Signature is not needed as all methods in Signature are
static, so just call the method.

[1] https://en.cppreference.com/w/cpp/memory/c/free
[2] https://stackoverflow.com/a/47368753

src/core/Alloc.cpp

@@ -17,6 +17,7 @@
 
 #include <QtGlobal>
 #include <cstdint>
+#include <cstdlib>
 #include <sodium.h>
 #if defined(Q_OS_MACOS)
 #include <malloc/malloc.h>

src/keeshare/ShareExport.cpp

@@ -149,8 +149,7 @@ namespace
             KeeShareSettings::Sign sign;
             auto sshKey = own.key.sshKey();
             sshKey.openKey(QString());
-            const Signature signer;
-            sign.signature = signer.create(bytes, sshKey);
+            sign.signature = Signature::create(bytes, sshKey);
             sign.certificate = own.certificate;
             stream << KeeShareSettings::Sign::serialize(sign);
             stream.flush();