Remove some compilation warnings.

llvm-svn: 157253
diff --git a/lld/lib/Core/YamlWriter.cpp b/lld/lib/Core/YamlWriter.cpp
index 03539c9..f3b3482 100644
--- a/lld/lib/Core/YamlWriter.cpp
+++ b/lld/lib/Core/YamlWriter.cpp
@@ -423,7 +423,7 @@
 
 private:
   // return a string of the correct number of spaces to align value
-  const char* spacePadding(int keyLen) {
+  const char* spacePadding(size_t keyLen) {
     const char* spaces = "                  ";
     assert(strlen(spaces) > keyLen);
     return &spaces[keyLen];