Mark extern definition of SDS_NOINIT in sds.h
Mark extern definition of SDS_NOINIT in sds.h

This helps in avoiding multiple definition of this variable, its also
defined globally in sds.c

Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

pmem/redis,pmem/redis,pmem/redis,pmem/redis