Add missing include.

Fixes the following compilation errors with gcc 4.7 and later.

messageserver.cpp: In static member function 'static void MessageServer::hupSignalHandler(int)':
messageserver.cpp:501:5: error: '::write' has not been declared
messageserver.cpp: In member function 'void MessageServer::handleSigHup()':
messageserver.cpp:508:5: error: '::read' has not been declared

Change-Id: I6fbcf2cba884f5e285c0259e020c60f4a44173b4
Reviewed-by: Valerio Valerio <valerio.valerio@jollamobile.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
libqmfclient: export a few private classes used by messageserver.

Since version 4.7.0, gcc constraints the ELF visibility of template
instantiations to the visibility of its template arguments (see
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35688).

Therefore all explicit instantiations of QPrivatelyImplemented<> in
qmailinstantiations.cpp are now hidden, because the template arguments
are private classes that have hidden visibility.

This commit restores pre-gcc-4.7 behavior by exporting the symbols
required to build messageserver successfully.

Change-Id: I7c0ae91a56cac390f1e73418af486578da665a94
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>