Create mysqlstorage_fix.h

because of error during compile replace void with bool on line 105

in mysqlstorage.cpp also replace it on initDbSession and add return true

log:
[ 86%] Building CXX object src/core/CMakeFiles/mod_core.dir/core.cpp.o
In file included from /home/trechber/quassel-branch_mysql_support/src/core/core.cpp:30:0:
/home/trechber/quassel-branch_mysql_support/src/core/mysqlstorage.h:105:16: error: conflicting return type specified for ‘virtual void MySqlStorage::initDbSession(QSqlDatabase&)’
In file included from /home/trechber/quassel-branch_mysql_support/src/core/mysqlstorage.h:24:0,
                 from /home/trechber/quassel-branch_mysql_support/src/core/core.cpp:30:
/home/trechber/quassel-branch_mysql_support/src/core/abstractsqlstorage.h:83:25: error:   overriding ‘virtual bool AbstractSqlStorage::initDbSession(QSqlDatabase&)’
make[2]: *** [src/core/CMakeFiles/mod_core.dir/core.cpp.o] Fehler 1
make[1]: *** [src/core/CMakeFiles/mod_core.dir/all] Fehler 2
make: *** [all] Fehler 2