folly/String-inl.h (humanify): avoid -Wsign-compare error

Summary:
* folly/String-inl.h (humanify): Count with a signed type.
Otherwise, gcc-4.9 fails with e.g.,
folly/String-inl.h:596:33: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
folly/String-inl.h:601:32: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo

Reviewed By: philipp@fb.com

Subscribers: trunkagent, folly-diffs@

FB internal diff: D1770451

Tasks: 5941250

Signature: t1:1770451:1420704354:c841ca453a5586fbf474535ca26cef17f95427dd

folly/String-inl.h

@@ -567,8 +567,8 @@ void backslashify(const String1& input, String2& output, bool hex_style) {
 
 template <class String1, class String2>
 void humanify(const String1& input, String2& output) {
-  int numUnprintable = 0;
-  int numPrintablePrefix = 0;
+  size_t numUnprintable = 0;
+  size_t numPrintablePrefix = 0;
   for (unsigned char c : input) {
     if (c < 0x20 || c > 0x7e || c == '\\') {
       ++numUnprintable;