nimble/host: Use static buffer for GAP mempool
nimble/host: Use static buffer for GAP mempool

It is only allocated on startup and host initialization will fail in
case it cannot be allocated, so let's just make it static.

X-Original-Commit: 881eae1633d347a9a30019e90ffe17585f467abf

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble