remove import of <asm-generic/errno.h> in src/bucket/InMemoryIndex.cpp (

#4636)

# Description

This import breaks builds on mac. I don't think the constants defined in
it are used?

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# Checklist
- [ ] Reviewed the
[contributing](https://github.com/stellar/stellar-core/blob/master/CONTRIBUTING.md#submitting-changes)
document
- [ ] Rebased on top of master (no merge commits)
- [ ] Ran `clang-format` v8.0.0 (via `make format` or the Visual Studio
extension)
- [ ] Compiles
- [ ] Ran all tests
- [ ] If change impacts performance, include supporting evidence per the
[performance
document](https://github.com/stellar/stellar-core/blob/master/performance-eval/performance-eval.md)

src/bucket/InMemoryIndex.cpp

@@ -10,7 +10,6 @@
 #include "util/types.h"
 #include "xdr/Stellar-ledger-entries.h"
 #include <algorithm>
-#include <asm-generic/errno.h>
 
 namespace stellar
 {