[clang-tidy] cpp: Use correct type for loop variable

Found with bugprone-too-small-loop-variable

Signed-off-by: Rosen Penev <rosenp@gmail.com>
diff --git a/src/Array.cpp b/src/Array.cpp
index 8838e5b..65ffaa7 100644
--- a/src/Array.cpp
+++ b/src/Array.cpp
@@ -61,8 +61,7 @@
 Array& Array::operator=(const PList::Array& a)
 {
     plist_free(_node);
-    for (unsigned int it = 0; it < _array.size(); it++)
-    {
+    for (size_t it = 0; it < _array.size(); it++) {
         delete _array.at(it);
     }
     _array.clear();
@@ -73,8 +72,7 @@
 
 Array::~Array()
 {
-    for (unsigned int it = 0; it < _array.size(); it++)
-    {
+    for (size_t it = 0; it < _array.size(); it++) {
         delete (_array.at(it));
     }
     _array.clear();