replace assert.h with cassert

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/lib/icu/CaseFold.cxx

@@ -40,9 +40,9 @@
 #include <windows.h>
 #endif
 
+#include <cassert>
 #include <memory>
 
-#include <assert.h>
 #include <string.h>
 
 AllocatedString<>

src/lib/icu/Collate.cxx

@@ -38,10 +38,10 @@
 #include <windows.h>
 #endif
 
+#include <cassert>
 #include <memory>
 #include <stdexcept>
 
-#include <assert.h>
 #include <string.h>
 
 #ifdef HAVE_ICU

src/lib/icu/Util.cxx

@@ -24,10 +24,10 @@
 
 #include <unicode/ustring.h>
 
+#include <cassert>
 #include <memory>
 #include <stdexcept>
 
-#include <assert.h>
 #include <string.h>
 
 AllocatedArray<UChar>