Fix building on Mac OS X 10.11 or older

[yan12125]

Type of change

• ✅ Bug fix (non-breaking change which fixes an issue)

Description and Context

• 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

Screenshots

N/A

Testing strategy

Build it! Some logs from build servers of MacPorts:

https://build.macports.org/builders/ports-10.9_x86_64-builder/builds/103349/steps/install-port/logs/stdio (Mac OS X 10.9)
https://build.macports.org/builders/ports-10.10_x86_64-builder/builds/100565/steps/install-port/logs/stdio (Mac OS X 10.10)

Checklist:

• ✅ I have read the CONTRIBUTING document. [REQUIRED]
• ✅ My code follows the code style of this project. [REQUIRED]
• ✅ All new and existing tests passed. [REQUIRED]
• ✅ I have compiled and verified my code with -DWITH_ASAN=ON. [REQUIRED]
• My change requires a change to the documentation, and I have updated it accordingly.
• I have added tests to cover my changes.

[droidmonkey]

Nice work!