databases/mariadb55-server: fix build with GCC-based architectures

Add USES=compiler:c++11-lang to fix:
/wrkdirs/usr/ports/databases/mariadb55-server/work/mariadb-5.5.64/storage/xtradb/include/os0sync.h:416:4: error: #error "Unsupported platform"

Approved by: mentors (implicit approval)