Avoid ambiguity which OUStringBuffer ctor to select

...for size_t (i.e., unsigned long long on Windows 64-bit), chosing among the
int, unsigned int, long, unsigned long overloads.  MSVC apparently happily picks
one of them, but clang-cl rightly complains.

Change-Id: Ib245925bdf9e4a34add6ca435a95559f4f07c4d7

fpicker/source/win32/filepicker/FileOpenDlg.cxx

@@ -38,9 +38,9 @@ namespace /* private */
     // OK button so that we determine the size of the text
     // currently in the edit field and resize our buffer
     // appropriately - in the future we will do this
-    const size_t MAX_FILENAME_BUFF_SIZE  = 32000;
-    const size_t MAX_FILETITLE_BUFF_SIZE = 32000;
-    const size_t MAX_FILTER_BUFF_SIZE    = 4096;
+    const sal_Int32 MAX_FILENAME_BUFF_SIZE  = 32000;
+    const sal_Int32 MAX_FILETITLE_BUFF_SIZE = 32000;
+    const sal_Int32 MAX_FILTER_BUFF_SIZE    = 4096;
 
     const LPCTSTR CURRENT_INSTANCE = TEXT("CurrInst");