Fix compiler warning in Subprocess

Summary: I'm going to have hhvm depend on Subprocess, found this build error in open source.  I figured the intent was to set r to the return code.  I could also delete the whole thing since it's unused.

Test Plan: Unit tests

Reviewed By: tudorb@fb.com

Subscribers: trunkagent, mwilliams, doug, njormrod

FB internal diff: D1583626

folly/Subprocess.cpp

@@ -493,7 +493,6 @@ int Subprocess::runChild(const char* executable,
                          char** argv, char** env,
                          const Options& options) const {
   // Now, finally, exec.
-  int r;
   if (options.usePath_) {
     ::execvp(executable, argv);
   } else {
@@ -818,4 +817,3 @@ Initializer initializer;
 }  // namespace
 
 }  // namespace folly
-