-Wstrict-prototypes warning when using CocoaPods

[rebello95]

When running pod spec lint on 0.5.0, the following warning is outputted:

• WARN | xcodebuild: SwiftGRPC/Sources/CgRPC/shim/cgrpc.h:206:63: warning: this function declaration is not a prototype [-Wstrict-prototypes]

[rebello95]

Fixed here: #295

It'd be great if we can get this in before the next release.