mariadb: disable -Werror

Otherwise i get the following errors for multibuild_c:32:

In file included from /var/tmp/paludis/build/dev-db-mariadb-5.5.33a-r1/work/mariadb-5.5.33a/storage/tokudb/ft-index/portability/toku_fair_rwlock.cc:96:0:
/var/tmp/paludis/build/dev-db-mariadb-5.5.33a-r1/work/mariadb-5.5.33a/storage/tokudb/ft-index/portability/toku_fair_rwlock.h: In function 'int toku_fair_rwlock_rdlock(toku_fair_rwlock_t*)':
/var/tmp/paludis/build/dev-db-mariadb-5.5.33a-r1/work/mariadb-5.5.33a/storage/tokudb/ft-index/portability/toku_fair_rwlock.h:119:48: error: integer overflow in expression [-Werror=overflow]
 #define RWS_QCOUNT_MASK (((1L<<RWS_QCOUNT_LEN)-1)<<RWS_QCOUNT_OFF)

Change-Id: Idbefc2f68896dbaa7eeacac5a295e5aa121c0ce7