Fix compilation of mysql db backend against MariaDB

MariaDB's connector-c by default provides mariadb_version.h instead of
mysql_version.h header, However, it's inderectly included through
mysql.h in both implementation: mariadb and mysql, so remove it's direct
inclussion from the source.

Signed-off-by: Alexander Golubev <fatzer2@gmail.com>

amarok/src/collectiondb.cpp

@@ -76,7 +76,6 @@
 
 #ifdef USE_MYSQL
     #include <mysql.h>
-    #include <mysql_version.h>
 #endif
 
 #ifdef USE_POSTGRESQL