AppleTLS: Build failure on Snow Leopard

```
/usr/bin/g++-4.2 -DHAVE_CONFIG_H -I. -I..  -Wall -I../lib -I../intl -DLOCALEDIR=\"/Users/adamv/homebrew/Cellar/aria2/1.17.0/share/locale\" -DCA_BUNDLE=\"/usr/local/share/ca-bundle.crt\" -DHAVE_CONFIG_H  -I../deps/wslay/lib/includes -I../deps/wslay/lib/includes -I/usr/include/libxml2 -F/usr/local/Frameworks  -Os -w -pipe -march=core2 -mmacosx-version-min=10.6 -c -o AppleTLSContext.o AppleTLSContext.cc
/usr/bin/g++-4.2 -DHAVE_CONFIG_H -I. -I..  -Wall -I../lib -I../intl -DLOCALEDIR=\"/Users/adamv/homebrew/Cellar/aria2/1.17.0/share/locale\" -DCA_BUNDLE=\"/usr/local/share/ca-bundle.crt\" -DHAVE_CONFIG_H  -I../deps/wslay/lib/includes -I../deps/wslay/lib/includes -I/usr/include/libxml2 -F/usr/local/Frameworks  -Os -w -pipe -march=core2 -mmacosx-version-min=10.6 -c -o AppleTLSSession.o AppleTLSSession.cc
AppleTLSContext.cc:52: error: ‘kSecClass’ was not declared in this scope
AppleTLSContext.cc:53: error: ‘kSecReturnRef’ was not declared in this scope
AppleTLSContext.cc:54: error: ‘kSecMatchPolicy’ was not declared in this scope
AppleTLSContext.cc:55: error: ‘kSecMatchLimit’ was not declared in this scope
AppleTLSContext.cc: In member function ‘bool aria2::AppleTLSContext::tryAsFingerprint(const std::string&)’:
AppleTLSContext.cc:180: error: ‘kSecClassIdentity’ was not declared in this scope
AppleTLSContext.cc:183: error: ‘kSecMatchLimitAll’ was not declared in this scope
AppleTLSContext.cc:193: error: ‘SecItemCopyMatching’ was not declared in this scope
```

Likely some missing includes. I'l dust of my Snow Leopard VM, /methinks

See: https://github.com/mxcl/homebrew/pull/19392#issuecomment-17917859