folly/io/Compression.cpp: avoid shadowing warnings for "rc"

folly/io/Compression.cpp: avoid shadowing warnings for "rc"

Summary:
gcc's -Wshadow (and the under-review -Wshadow-local) would evoke this:
```
folly/io/Compression.cpp:650:9: error: declaration of 'rc' shadows a previous local [-Werror=shadow-compatible-local]
folly/io/Compression.cpp:637:7: error: shadowed declaration is here [-Werror=shadow-compatible-local]
folly/io/Compression.cpp:566:9: error: declaration of 'rc' shadows a previous local [-Werror=shadow-compatible-local]
folly/io/Compression.cpp:548:7: error: shadowed declaration is here [-Werror=shadow-compatible-local]
```
In each case, I removed the "int" from the latter declaration.

Reviewed By: philippv

Differential Revision: D3966308

fbshipit-source-id: 584cb9ffe8ba0e56914223c440efabe9e0de6b17