cmFileCommand: Fix signature of CURLOPT_DEBUGFUNCTION callback

Curl documents that the callback should return type `int`.

Source/cmFileCommand.cxx

diff --git a/Source/cmFileCommand.cxx b/Source/cmFileCommand.cxx
index e4728ac..4a65237 100644
--- a/Source/cmFileCommand.cxx
+++ b/Source/cmFileCommand.cxx
@@ -1610,8 +1610,8 @@
   return realsize;
 }
 
-size_t cmFileCommandCurlDebugCallback(CURL*, curl_infotype type, char* chPtr,
-                                      size_t size, void* data)
+int cmFileCommandCurlDebugCallback(CURL*, curl_infotype type, char* chPtr,
+                                   size_t size, void* data)
 {
   cmFileCommandVectorOfChar& vec =
     *static_cast<cmFileCommandVectorOfChar*>(data);