Don't always import Protobuf like a framework

If the macro isn't set, e.g. for Bazel compilations, import the way it was done before fb4a54541a.

src/objective-c/ProtoRPC/ProtoRPC.m

@@ -33,7 +33,11 @@
 
 #import "ProtoRPC.h"
 
-#import <Protobuf/GPBProtocolBuffers.h>
+#if GPB_USE_PROTOBUF_FRAMEWORK_IMPORTS
+ #import <Protobuf/GPBProtocolBuffers.h>
+#else
+ #import <GPBProtocolBuffers.h>
+#endif
 #import <RxLibrary/GRXWriteable.h>
 #import <RxLibrary/GRXWriter+Transformations.h>